Current measurement

ABSTRACT

The present invention relates to current measurement apparatus  100 . The current measurement apparatus  100  comprises a measurement arrangement  110, 114  which is configured to be disposed in relation to a load  108  which draws a current signal, the measurement arrangement being operative when so disposed to measure the load drawn current signal. The current measurement apparatus  100  also comprises a signal source  112  which is operative to apply a reference input signal to the measurement arrangement  110, 114  whereby an output signal from the measurement arrangement comprises a load output signal corresponding to the load drawn current signal and a reference output signal corresponding to the reference input signal. The current measurement apparatus  100  further comprises processing apparatus  116  which is operative to receive the output signal and to make a determination in dependence on the reference output signal and the load output signal, the determination being in respect of at least one of the load drawn current signal and electrical power consumed by the load.

This application is a Continuation-In-Part of U.S. application Ser. No.13/672,236, filed on Nov. 8, 2012, which claims the priority of U.S.Provisional Application No. 61/557,369, filed on Nov. 8, 2011. Thisapplication is a Continuation-In-Part of International Application No.PCT/GB2012/052251, filed on Sep. 12, 2012, which claims the priority ofGreat Britain Application No. 1115648.6, filed on Sep. 12, 2011, U.S.Provisional Application No. 61/557,369, filed on Nov. 8, 2011, and GreatBritain Application No. 1207905.9, filed on May 4, 2012. Thisapplication is a Continuation-In-Part of U.S. application Ser. No.14/344,052, which is a 371 national phase entry of InternationalApplication No. PCT/GB2012/052251, filed on Sep. 12, 2012, which claimsthe priority of Great Britain Application No. 1115648.6, filed on Sep.12, 2011, U.S. Provisional Application No. 61/557,369, filed on Nov. 8,2011, and Great Britain Application No. 1207905.9, filed on May 4, 2012.These documents are fully incorporated by reference as if fully setforth herein. All publications referenced herein are fully incorporatedby reference, as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to current measurement apparatus andmethods of measuring current, in particular but not exclusively formeasuring high levels of current present in electricity consumptioncircuits, electricity generation circuits and the like. The presentinvention further relates to apparatus, such as electricity generation,transmission, distribution or consumption apparatus, which comprisessuch current measurement apparatus or is operative to measure currentaccording to such methods. In particular but not exclusively linevoltage is measured in conjunction with current to provide fordetermination of at least one of electrical power and power quality.

BACKGROUND ART

A current shunt provides for indirect measurement of current values bythe measurement of the voltage developed across the shunt by the currentpassing through the shunt. Typical applications for current shuntsinclude electricity usage control, over-current protection and meteringof electricity consumption and generation. In use a shunt of knownresistance is provided in series with a load and the voltage developedacross the shunt by the load drawn current is measured. The currentpassing through the shunt is then determined on the basis of Ohm's Lawin view of the measured voltage and the known resistance of the shunt.

Certain applications, such as metering of electricity consumption andgeneration, require measurement to high accuracy over extended periodsof time. For example in North America the ANSI C12.20 standard specifiesan accuracy of ±0.5% for Class 0.5 consumption meters and ±0.2% forClass 0.2 consumption meters. Standards applicable in Europe andelsewhere, such as IEC 62053, specify similar accuracy requirements. Itcan therefore be appreciated that the resistance of the shunt must beknown to high precision to enable a meter to meet regulated accuracyrequirements. Although the shunt resistance is normally low to minimisepower dissipation and undesirable circuit effects, the shunt isnevertheless liable to heating with temperature drift giving rise to achange in resistance which may cause a loss of measurement accuracy in ashunt of ordinary temperature coefficient of resistance. Shunt resistorsformed from manganin alloy are therefore widely used in view of theirvery low temperature coefficient of resistance. It may also be apparentthat accurate current measurement depends on measurement of the voltagedeveloped across the shunt being accurate and stable with temperatureand lifetime. This is because a change in the transfer gain or lack ofprecision in references used in the voltage measurement circuit willcause an error. It is normal for these reasons to perform a one-offfactory calibration when the shunt and the readout electronics arecombined so that a factor related to the actual combined transferfunction for current to measurement value, which is determined largelyby the shunt resistor and voltage measurement, can be stored and used insubsequent measurements to achieve the desired precision.

An alternative known approach to measuring high values of currentinvolves the use of a current transformer wound on a core, which isdisposed around a conductor carrying current to be measured. The currenttransformer has the advantages over the shunt resistor of being lessinvasive and providing for isolation from the current carryingconductor. However the current transformer is capable of measuring ACcurrent only. The current transformer generates a current in thesecondary coil, which is a ratio of the current in the primaryconductor, and the secondary coil current is then turned into a voltageby a load, known as a burden resistor. Accurate measurement of thevoltage across the burden resistor combined with accurate knowledge ofthe transfer function of the primary current to voltage across theburden resistor (i.e. combining the effect of number of turns, themagnetic and the burden resistor) are needed to accurately and preciselymeasure the current. As with the current shunt, one-off factorycalibration is often performed to compensate for inaccuracies in some orall of the elements that contribute to the overall transfer function ofprimary current to measurement value.

In contrast another approach uses a Hall current probe which is capableof measuring both AC and DC. In an open loop configuration the Hallcurrent probe is, however, liable to non-linearity and temperaturedrift. In a closed loop configuration the Hall current probe provides animprovement with regards to non-linearity and temperature drift althoughthe weight and size of the configuration increases significantly wherehigher currents are measured. It is further known to use the Rogowskicoil current probe to measure high levels of current. Most knownapproaches to current measurement, such as by way of the shunt resistor,the current transformer, the Rogowski coil and the Hall current probe,are described and discussed in Current Sensing Techniques: A Review,Silvio Ziegler Robert C. Woodward and Herbert Ho-Ching Iu, IEEE SensorsJournal, Vol. 9, No. 4, April 2009. The different known approaches havetheir respective advantages and disadvantages.

Load current measurement is often made in conjunction with line voltagemeasurement, which involves measuring the voltage between the conductorsover which the current is delivered, in order to determine theelectrical power. As the line voltage is often many times larger thanthe largest signal that can be safely or conveniently measured, the mostcommon means of measuring the line voltage is through a resistivepotential divider between the conductors, which lowers the voltage to bemeasured by the factor of the divider ratio. The divider ratio and theaccuracy of the subsequent voltage measurement chain needs to be knownand be sufficiently stable to meet the accuracy requirements of thepower measurement application; accuracy requirements for powermeasurement are specified in the aforementioned standards. Accurate linevoltage measurement normally depends on the use of components with goodtemperature coefficients and known values and on factory calibration,amongst other techniques.

High accuracy power calculation also requires accurate and stablerelative phase and frequency response of the load current and the linevoltage measurements in order to accurately determine such metrics anddifferences between active and reactive power, power factor and harmoniccontent amongst others.

The present inventor has become appreciative of the various shortcomingsof known approaches to current measurement and power measurement, suchas the approaches described in outline in the preceding paragraphs. Itis therefore an object for the present invention to provide improvedcurrent measurement apparatus which is configured to provide foraccurate measurement of current, for example in a circuit carrying mainscurrent. It is another object for the present invention to provide animproved method of measuring current which provides for accuratemeasurement of current, for example in a circuit carrying mains current.It is a further object for the present invention to provide improvedvoltage measurement apparatus which is configured to provide foraccurate measurement of line voltage, such as in a circuit carryingmains current, whereby accurate power measurement may be achieved. It isa yet further object for the present invention to provide an improvedmethod of measuring voltage which provides for accurate measurement ofline voltage, such as in a circuit carrying mains current, wherebyaccurate power measurement may be achieved.

STATEMENT OF INVENTION

The present invention has been devised in the light of the inventors'appreciation of the shortcomings of known approaches to currentmeasurement. Therefore and according to a first aspect of the presentinvention there is provided current measurement apparatus comprising:

-   -   a measurement arrangement configured to be disposed in relation        to a load which draws a current signal, the measurement        arrangement being operative when so disposed to measure the load        drawn current signal;    -   a signal source operative to apply a reference input signal to        the measurement arrangement whereby an output signal from the        measurement arrangement comprises a load output signal        corresponding to the load drawn current signal and a reference        output signal corresponding to the reference input signal; and    -   processing apparatus which is operative to receive the output        signal and to make a determination in dependence on the        reference output signal and the load output signal, the        determination being in respect of at least one of the load drawn        current signal and electrical power consumed by the load.

In use the measurement arrangement is disposed in relation to the load,such as by disposing the measurement arrangement in relation to anelectrical conductor through which the current signal is drawn by theload. The measurement arrangement may be disposed close to or in serieswith the electrical conductor. The load, for example, may be anelectrical appliance in a domestic setting with the electrical appliancebeing operative to draw the current signal though an electricalconductor, such as a conductor forming part of a primary circuit in anelectricity consumption meter. The current measurement apparatus maytherefore comprise an electrical conductor through which the currentsignal is drawn by the load and may also comprise the load.

Accurate measurement of load drawn current, e.g. for the purpose ofmeasurement of power consumption, may be achieved in dependence on theload output signal alone when the transfer function of the measurementarrangement is accurately known and stable over time. However thetransfer function of the measurement arrangement is liable to be unknownat point of assembly due to manufacturing tolerances. Also the transferfunction is liable to change over time and thereby give rise toinaccuracy in determination of the load drawn current signal. Thepresent invention may obviate the need to determine the transferfunction at point of assembly, e.g. by way of a calibration approach, ormay obviate the need to configure the measurement arrangement tominimise drift in the transfer function. The present invention does soby applying a reference input signal by way of the signal source to themeasurement arrangement as the current signal is drawn by the loadthrough the measurement arrangement. The signal source may be configuredto apply the reference input signal in the form of charge, voltage orcurrent itself. The measurement arrangement is operative to provide anoutput signal comprising a load output signal corresponding to the loaddrawn current signal and a reference output signal corresponding to thereference input signal. The processing apparatus is operative to receivethe output signal and to make a determination in respect of at least oneof the load drawn current signal and electrical power consumed by theload. The determination is made in dependence on the reference outputsignal and the load output signal. According to one example theprocessing apparatus may determine the load drawn current in dependenceon the load output signal, the reference output signal and the referenceinput signal. According to this example the processing apparatus may beoperative to determine at least a part of the transfer function for themeasurement arrangement in dependence on the reference output signal andthe reference input signal and then to determine the load drawn currentin dependence on the load output signal and the transfer function. Theprocessing apparatus is therefore operative to make a determination independence on the reference input signal as well as the reference outputsignal and the load output signal. By way of further example theprocessing apparatus may determine a change in the load drawn currentfrom an earlier determined load drawn current in dependence on a changein the load output signal from an earlier determined load output signaland on a change in the reference output signal from an earlierdetermined reference output signal. By way of another example a changein electrical power consumption from an earlier determined electricalpower consumption may be determined in dependence on a change in theload output signal, on a change in the reference output signal and on ameasured line voltage or a measured change in line voltage.Alternatively the power consumption itself may be determined independence on the load output signal, the reference output signal, thereference input signal and the measured line voltage.

Making a determination in respect of the load drawn current signal independence on the reference output signal and the load output signal mayaddress whatever change in the transfer function of the measurementarrangement there may be by relying on the reference signal being knownor being determinable to a required accuracy and on accuracy ofdetermination of the reference output signal. Accuracy of determinationin respect of the load drawn current signal may therefore not becompromised by a change in the transfer function of a measurementarrangement comprising, for example, a current sensor and circuitryoperative to acquire signals from the current sensor. The processingapparatus may be operative to determine a relationship between thereference input signal and the reference output signal. In addition theprocessing apparatus may be operative to adjust determination of theload drawn current on the basis of the load output signal in dependenceon the relationship between the reference input signal and the referenceoutput signal to thereby correct for inaccuracy, e.g. in the currentsensor and its acquisition circuitry. Determining the relationshipbetween the reference input signal and the reference output signal maycomprise determining at least one characteristic of the transferfunction of the measurement arrangement, such as resistance in a simpleexample where the measurement arrangement comprises a current sensor inthe form of a shunt resistor with no appreciable frequency dependency.The processing apparatus may be operative to determine a change in atleast one characteristic of the transfer function over time. In additionthe processing apparatus may be operative to adjust determination of theload drawn current on the basis of the load output signal in dependenceon the change in the at least one characteristic. For example theprocessing apparatus may be operative to acquire first and secondreference output signals at different times and to adjust thedetermination of the load drawn current on the basis of the load outputsignal in dependence on a difference between the first and secondreference output signals. The processing apparatus may be operative toadjust the determination of the load drawn current on the basis of atransfer function, such as a more recently determined transfer function.Alternatively or in addition the processing apparatus may be operativeto compare a change in transfer function, such as a change determined bythe difference between the first and second reference output signals,with a threshold value. More specifically the processing apparatus maybe operative to adjust the determination of the load drawn current ifthe change is greater than the threshold value.

The measurement arrangement may comprise a current sensor which isoperative to sense the load drawn current signal and the reference inputsignal. The measurement arrangement may comprise a transimpedancecircuit whereby sensed current may be converted to a correspondingvoltage. In one form the transimpedance circuit may be comprised in thecurrent sensor. The current sensor may be operative of itself as atransimpedance circuit. For example the current sensor may be a shuntresistor which is operative of itself to sense current and provide acorresponding measureable voltage. The current sensor may be configuredto be disposed in series with the load. More specifically the currentsensor may be configured to be disposed such that the load draws currentthrough the current sensor. In another form the transimpedance circuitand the current sensor may be different components. For example thecurrent sensor may be a current transformer and the transimpedancecircuit may be a burden resistor. The current sensor may be configuredto be disposed in relation to the load whereby the load drawn currentsignal electromagnetically induces an induced current signal in thecurrent sensor. The current measurement apparatus may be configured suchthat the signal source is operative to at least one of: pass thereference input signal through a path followed by the load drawn currentsuch that the reference input signal is sensed by the current sensor;and electromagnetically induce an induced reference signal in thecurrent sensor. Either approach to applying the reference input signalmay be used with a current sensor which is operative such that the loaddrawn current signal is either drawn through the current sensor orinduces an induced current signal in the current sensor. Where a shuntresistor is used the overall transfer function from the load drawncurrent to the output signal from the measurement arrangement mayinclude characteristics, amongst others, of the shunt resistor itself,of signal conditioning circuitry and of signal acquisition circuitrywith one or more of these characteristics being common to the referenceinput signal and the load drawn current signal. Where a currenttransformer is used the overall transfer function from the load drawncurrent to the output signal from the measurement arrangement mayinclude characteristics, amongst others, of the electromagnetic couplingincluding alignment, of the current transformer, of the burden impedanceand of the signal acquisition circuitry with one or more of thesecharacteristics being common to the reference input signal and the loaddrawn current signal.

The measurement arrangement may be configured to acquire a signal fromthe current sensor, such as a voltage signal developed across a shuntresistor by the load drawn current signal and the reference inputsignal. Circuitry to acquire the signal may be configured to perform atleast one of gain, filtering, sampling, analogue to digital conversionand reconstruction. The processing apparatus may be operative: todetermine, upon application of the reference input signal, a factorcorresponding to a change in the transfer function of the measurementarrangement; and to make the determination in respect of the load drawncurrent signal in dependence on the factor. The factor may relate to achange over time in the reference output signal. For example thereference output signal may be acquired after calibration and stored.Subsequently the reference output signal may be acquired again when thetransfer function of the measurement arrangement has changed. The factormay characterise the difference between the stored reference outputsignal and the presently determined reference output signal. The loaddrawn current signal as determined on the basis of the load outputsignal may then be changed in dependence on the factor to address thechange in transfer function.

The transfer function of the measurement arrangement may be liable todrift, e.g. on account of temperature change or as a consequence ofelectromigration. The signal source may therefore be operative and theprocessing apparatus may be operative to make a determination withregards to the transfer function or change in transfer function on anintermittent basis, whereby the transfer function or change isdetermined less frequently than in respect of the load drawn currentsignal. More specifically the signal source and the processing apparatusmay be operative to determine the transfer function or change intransfer function at regular or irregular intervals. Where the intervalsare regular the length of an interval may be determined on the basis ofwhen re-calibration is likely to be required having regards to, forexample, the anticipated effect of electromigration over time.Alternatively or in addition at least one of the transfer function orchange in transfer function may be determined on an irregular basis independence on a determined condition of the current measurementapparatus, e.g. where there is an undue change in temperature whichmight be liable to cause an inaccuracy inducing impedance drift.Apparatus according to the invention may therefore obviate the need forexternal calibration beyond calibration upon installation or at thepost-manufacture stage or allow for less frequent external calibration.When no reference input signal is being applied to the measurementapparatus the load drawn current is determined on the basis of the loadoutput signal and the transfer function as most recently determined orthe factor representing the most recent change in the transfer function.

The reference input signal may have a known characteristic, e.g. apredetermined characteristic which is stored, such as a frequencyprofile, which enables the processing apparatus to extract the referenceoutput signal from the output signal from the measurement arrangement.For example the known characteristic may comprise one or more frequencyor phase components which of the load drawn current and reference inputsignals are substantially unique to the reference input signal andtherefore susceptible to filtering or frequency analysis. Application ofthe reference input signal to the measurement arrangement may give riseto a signal having a recognisable characteristic in the measurementarrangement, with the recognisable characteristic corresponding to theknown characteristic of the reference input signal. The processingapparatus may be operative to perform the extraction in dependence uponthe recognisable characteristic of the signal.

The load drawn current and reference input signals may differ from eachother, e.g. in respect of their respective frequency profiles, tothereby enable the processing apparatus to separate the load outputsignal and the reference output signal from each other. For example theload drawn current signal may be a mains signal consisting essentiallyof a 50 Hz or 60 Hz frequency component and the reference input signalmay be an alternating or pulsed signal having one or more frequency,phase or modulation components outside the band of the mains signal.Alternatively and where the load draws current from a direct currentsource the load drawn current signal may be substantially a directcurrent signal and the reference input signal may be an alternating orpulsed or modulated signal. According to a further example the loaddrawn current signal may be an alternating signal and the referenceinput signal may consist essentially of direct current. According to ayet further example the load drawn current signal may be a mains signaland the reference input signal may comprise at least one frequency,phase or modulated component substantially absent from the load drawncurrent signal. The signal source may therefore be operative to apply areference input signal having a known characteristic, which issubstantially absent from the load drawn current signal, to themeasurement arrangement. The processing apparatus may be operative tolearn which known characteristic in the load drawn current issubstantially absent from the load drawn current signal and to configurethe reference input signal accordingly. For example the processingapparatus may be operative ordinarily such that the reference inputsignal comprises a 100 Hz signal and may be operative in dependence on100 Hz noise being present in the load drawn current signal to changethe reference input signal such that it comprises an 83 Hz signal.Similarly the current measurement apparatus may be operative such thatthe reference input signal is locked in phase or frequency relative tothe fundamental frequency of the load drawn current so as to be in aclean part of either the frequency or time domain. For example thereference input signal may be locked such that it is offset with respectto a multiple of the measured mains frequency or such that it holds aposition spaced apart from the zero crossing point of the load drawnsignal or the line frequency. Similarly the current measurementapparatus may be operative such that the reference input signal ismodulated at the fundamental frequency or a sub-harmonic of the loaddrawn current, for example to be either present or not present during atleast part of the complete fundamental cycle of the load drawn currentsignal. Additionally at least one of the signal source, signalacquisition and processing apparatus may be configured to align to arelated frequency or phase of the load drawn signal. The processingapparatus may be further configured to separate the load output andreference output signals from each other in dependence on the knowncharacteristic.

As mentioned above accurate current measurement according to the presentinvention relies on the reference input signal. The signal source maytherefore comprise a signal source reference circuit and the currentmeasurement apparatus may be configured such that the reference inputsignal is set by the signal source reference circuit. Furthermore andaccording to a first approach the current measurement apparatus mayfurther comprise a reference signal reference circuit. The referencesignal reference circuit may be of the same form as a current sensorcomprised in the measurement arrangement, e.g. a passive component wherethe current sensor is a passive component, such as a resistor when thecurrent sensor is a shunt resistor. A characteristic, such as impedance,of the reference signal reference circuit may be known to a requiredaccuracy. The current measurement apparatus may be operative to storethe known characteristic of the reference signal reference circuit.Furthermore the reference signal reference circuit may be configured tomaintain accuracy according to requirements, such as over a specifiedperiod of time and/or within a range of operating temperatures. Thecurrent measurement apparatus may be configured such that themeasurement arrangement is operative to acquire a reference voltagesignal developed across the reference signal reference circuit solely independence on application of the reference input signal to the referencesignal reference circuit by the signal source. The current measurementapparatus may therefore be configured to determine the load drawncurrent signal in dependence on: the known characteristic, e.g.impedance, of the reference signal reference circuit; the referencevoltage signal; the reference output signal; and the load output signal.Alternatively the current measurement apparatus may be operative todetermine, i.e. ascertain, the reference input signal in dependence onthe acquired reference voltage signal and the known characteristic ofthe reference signal reference circuit and to thereafter determine theload drawn current signal in dependence on the reference output signal,the load output signal and the ascertained reference input signal.

The current measurement apparatus may be configured such that themeasurement arrangement is operative to acquire the reference voltagesignal on a periodic basis, whereby the reference voltage signal isdetermined less frequently than the load drawn current signal. Forexample one reference voltage signal may be acquired for every onehundred or one thousand acquisitions by the measurement arrangement. Thecurrent measurement apparatus may comprise a switch arrangement which isoperative to switch the acquisition circuitry comprised in themeasurement arrangement and the signal source between a current sensorcomprised in the measurement arrangement and the reference signalreference circuit. A time between acquisitions of subsequent referencevoltage signals may depend on an extent to which the reference inputsignal changes over time, i.e. a stability of the reference inputsignal. For example if the reference input signal is less stable morefrequent acquisition of the reference voltage signal may be required.Although the signal source reference circuit may have a stabilityrequirement, an absolute value such as a current value of the referenceinput signal may have no bearing on measurement accuracy. This isbecause measurement accuracy may rely on the accurately known referencesignal reference circuit. However the reference signal reference circuitnormally carries a lower level of current than the current sensor, e.g.mA versus Amps, and is therefore less liable to degradation and loss ofaccuracy than the current sensor. The reference signal reference circuitmay also be formed from material having superior time and temperaturedrift performance in view of the reference signal reference circuitnormally carrying a lower level of current than the current sensor.Furthermore a reference signal reference circuit, such as a resistor,capable of maintaining accuracy may be provided more readily and henceat lower cost than a current sensor capable of maintaining the samelevel of accuracy. The present approach of relying on the referencesignal reference circuit may address absolute and relative errors causedby the measurement arrangement and the signal source. For example andwhere the measurement arrangement comprises signal conditioningcircuitry, analogue to digital conversion circuitry and signalprocessing circuitry the present approach may address errors contributedby all of this circuitry.

Acquisition of the reference voltage signal may result inunder-measurement of the load drawn current signal. Where the frequencyof reference voltage signal acquisition is sufficiently low an extent ofunder-measurement of the load drawn current signal may be withinacceptable bounds. For example if the reference voltage signal isacquired no more than once every one thousand cycles the measurementerror may be less than a desired figure of −0.1%. Where the measurementerror is higher than acceptable the current measurement apparatus may beconfigured to address this problem. More specifically the currentmeasurement apparatus may be configured to determine a signal across thecurrent sensor which was missed when acquiring a reference voltagesignal from the reference signal reference circuit. The missed signalmay be determined by the processing apparatus, e.g. by interpolation, independence on previous and subsequent signals acquired by themeasurement arrangement.

The reference signal reference circuit may be formed from at least onediscrete component. Alternatively the reference signal reference circuitmay be comprised in an integrated circuit. Where the reference signalreference circuit is comprised in an integrated circuit the currentmeasurement apparatus may be configured to change a reference providedby the reference signal reference circuit, e.g. by way of a referenceadjustment arrangement, such as a trimmer as described elsewhere herein.The current measurement apparatus may further comprise at least onetemperature sensor. The temperature sensor may be operative to sense atemperature at or in the vicinity of the reference signal referencecircuit. Furthermore behaviour of the reference signal reference circuitwith changing temperature may be determined by a calibration procedure.The determined behaviour may be stored in the current measurementapparatus. The current measurement apparatus may be configured tocompensate for changing temperature in dependence on sensed temperatureand the determined behaviour by means described elsewhere herein, e.g.by application of a transform which may be in the form of a lookuptable. Temperature change may cause the reference current to drift.Therefore a temperature sensor may be operative to sense a temperatureof the current measurement apparatus and the current measurementapparatus may be configured to change a determined load drawn currentsignal in dependence on a change in sensed temperature. Alternatively orin addition the current measurement apparatus may be configured tochange the reference input signal itself, a determined transfer functionor a determined change in the transfer function. Configuration may be byway of application of a transform, such as may be stored as a lookuptable, as is described elsewhere herein.

According to a second approach the current measurement apparatus may beconfigured such that the reference input signal is predetermined, e.g.by setting the reference input signal at a desired current value, orascertained, e.g. by measuring the reference input signal anddetermining the load drawn current signal accordingly. The referenceinput signal may be determined solely by the signal source referencecircuit. The signal source reference circuit may comprise at least oneof a passive reference circuit, such as a precision resistor, and anactive reference circuit, such as a reference circuit comprising a bandgap reference. The signal source reference circuit may be external to amain part of the current measurement apparatus. For example the mainpart of the current measurement apparatus may be constituted as aprinted circuit board arrangement, multi-chip module, integrated circuitor the like with the signal source reference circuit being an externalcomponent, such as a temperature stabilised precision resistor. Theactive reference circuit may be comprised in an integrated circuitfurther comprising at least another part of the current measurementapparatus. Drift, such as might be caused by electromigration ortemperature variation, is normally less where current levels are lower.The signal source reference circuit may be therefore less liable todrift than the current sensor because the latter normally carries ahigher level of current.

The signal source reference circuit may be liable to degrade over timewith the reference input signal suffering a consequential loss ofaccuracy. Therefore the current measurement apparatus may comprise asecond signal source reference circuit, e.g. a second passive referencecircuit such as a resistor, and the current measurement apparatus may beconfigured to determine a difference between a reference provided independence on the first signal source reference circuit and a referenceprovided in dependence on the second signal source reference circuit.The difference may be determined on a periodic basis, e.g. once a day oronce a week, whereby the difference is determined less frequently thanthe load drawn current signal. The current measurement apparatus may beconfigured to switch between the first and second signal sourcereference circuits. Furthermore the current measurement apparatus may beoperative to change the determined load drawn current signal independence on the determined difference. The determined load drawncurrent signal may be changed by changing the reference input signal, afactor corresponding to a change in transfer function of the measurementarrangement, a transfer function determined for the measurementarrangement or the determined load drawn current signal itself. Thechange may be effected by one or more of the approaches describedherein, such as by application of a transform stored as a lookup table.The second signal source reference circuit may be less liable todegradation than the first signal source reference circuit because theformer is operative less frequently than the first signal sourcereference circuit, e.g. the second signal source reference circuit maycarry current only once every two weeks. Therefore the referenceprovided by the second signal source reference circuit may be lessliable to change from a known value than the reference provided by thefirst signal source reference circuit. Determination of a differencebetween the first and second signal source references having regards totheir earlier known values may provide for configuration of the currentmeasurement apparatus to compensate for degradation of the first signalsource reference circuit. This approach may be suited to forms of theinvention in which the first and second signal source reference circuitsare accurately matched at the outset, e.g. where the first and secondreferences are provided in dependence on first and second resistorsformed on a printed circuit board or on first and second capacitorsformed in an integrated circuit.

The current measurement apparatus may comprise at least one temperaturesensor. The temperature sensor may be disposed so as to sense atemperature of a signal source reference circuit or a temperature in thevicinity of a signal source reference circuit. More specifically thesignal source may be operative to change at least one of the referenceinput signal and the determined load drawn current signal in dependenceon an output from the temperature sensor to thereby compensate fortemperature drift. Behaviour of the current measurement apparatus as awhole or in part, e.g. in respect of a signal source reference circuit,in response to temperature change may be determined during an initialcalibration procedure and the current measurement apparatus may beconfigured accordingly. Compensation for temperature drift may beeffected by way of application of a transform, which characterisestemperature behaviour, to the determined load drawn current signal. Thetransform may have the form of an algorithm, which is executed atruntime, or a lookup table.

The signal source reference circuit may be operative to provide areference, e.g. a reference voltage or reference resistance, whereby thereference input signal depends on the reference. The reference may bepredetermined, i.e. the reference may be set at a desired value, wherebythe signal source applies a predetermined reference input signal to themeasurement arrangement. Alternatively the reference may not bepredetermined or may be predetermined within bounds of insufficientaccuracy with the reference input signal being ascertained, e.g. bymeasurement during an external calibration procedure, and the processingapparatus being configured accordingly. Irrespective of whether or notthe reference is predetermined the signal source reference circuit maybe configured to maintain accuracy of the reference to within limitssufficient to provide a required degree of accuracy of currentmeasurement. For example where an accuracy of current measurement of±0.2% is required the signal source reference circuit may be configuredsuch that the reference maintains accuracy to within ±0.02% over atemperature range, such as −20 to 85 degrees Celsius, and for a minimumperiod of time. Where the reference is predetermined the signal sourcereference circuit may be configured for alteration of the reference. Thereference may not be of a desired value following manufacture or thereference may drift from a desired value over a period of time. Thereference may therefore be set at a desired value to a required degreeof accuracy, e.g. by a calibration procedure which is carried out at theoutset or at a later stage. The signal source reference circuit maycomprise a reference adjustment arrangement, such as a trimmer, which isoperative to change the reference. The reference adjustment arrangementmay be an active or passive circuit. The current measurement apparatusmay comprise data storage, which is operative to store adjustment data,and the reference adjustment arrangement may be operative in dependenceon the stored adjustment data. The adjustment data may be determined independence on the calibration procedure. The data storage may benon-volatile memory, such as one time programmable memory.

Alternatively or in addition and where the reference input signal isascertained, e.g. by measurement of a reference value of the signalsource reference circuit or of the reference input signal itself, theprocessing apparatus may be configured to take account of theascertained, e.g. accurately known, reference input signal. There may beno need to set the reference provided by the signal source referencecircuit at a desired value provided the reference input signal is knownaccurately. Alternatively the reference may have suffered drift from adesired value. The current measurement apparatus may therefore comprisedata storage, which is operative to store adjustment data correspondingto the determined reference value or reference input signal, and theprocessing apparatus may be operative to determine the load drawncurrent signal in dependence on the adjustment data.

Calibration may, for example, involve measuring the reference inputsignal by reference measuring apparatus, which is already calibrated andstabilised to a required standard. As described above the signal sourcereference circuit or the processing apparatus may then be configured inaccordance with the measured reference input signal to either alter thereference value of the reference circuit or to change how the load drawncurrent signal is determined. Such a calibration procedure may beconfigured to address other sources of inaccuracy in the currentmeasurement apparatus, such as in temperature measuring apparatus, clockcircuitry, signal conditioning circuitry and analogue-to-digitalconversion circuitry.

The reference input signal may comprise plural frequency components. Thesignal source may therefore be configured such that the reference inputsignal comprises at least one of: a frequency that changes over time,e.g. changes progressively between a first value and a second value;different frequencies at any one time; and frequency components whichare out of phase with each other. Having out of phase frequencycomponents may be advantageous: with regards to the ease with which thereference output signal is extracted from the output from themeasurement arrangement; and where addressing a frequency dependency ofthe measurement arrangement. Having at least one of a frequency thatchanges over time and different frequencies at any one time may providefor change from one operative frequency to another to, for example,avoid noise or a frequency trap and may also be advantageous withregards to addressing a frequency dependency of the measurementarrangement, such as self inductance. A frequency dependency, such asself inductance, may give rise to a difference between the transferfunction seen by the reference input signal and the transfer functionseen by the load drawn current signal. The current measurement apparatusmay therefore be operative to determine a frequency dependent transferfunction of the measurement arrangement over a range of frequencies independence on the reference input signal comprising at least one of: afrequency that changes over time; and different frequencies at any onetime. This step may be carried out upon initial calibration or on aperiodic basis when it may be deemed appropriate to further determine afrequency dependent transfer function, which may be stored by thecurrent measurement apparatus. The current measurement apparatus may befurther operative to modify, in dependence on the determined frequencydependent transfer function, the load drawn current signal determined inthe absence of application of the reference input signal. The currentmeasurement apparatus may thereby be operative to take account of afrequency dependency of the measurement arrangement. The processingapparatus may be operative such that the frequency dependency of themeasurement arrangement may comprise at least one characteristic ofamplitude, phase and group delay response with frequency.

The signal source may be configured to change an amplitude of thereference input signal. Changing an amplitude of the reference inputsignal may be advantageous where the load drawn current signal changesbetween large and small values, e.g. to achieve a compromise betweenaccuracy and power consumption. In certain applications, such as indomestic supply of electricity, the load drawn current signal may varybetween 0 and 50 Amps. Changing an amplitude of the reference inputsignal may also be advantageous to trade-off power consumption and timeinvolved in acquiring data for determining different transfer functions.For example acquisition apparatus may operate at a higher rate duringfactory calibration to provide for faster determination of transferfunction to the required accuracy. By way of another example theacquisition apparatus may operate at a lower rate when monitoring forchange in transfer function for diagnostic purposes to thereby save onpower consumption. Where the reference input signal comprises differentfrequency components at any one time, amplitudes of the differentfrequency components may differ from each other. The signal source maybe configured to generate such a reference input signal.

The current measurement apparatus may be configured to change adirection in which the reference input signal is passed through themeasurement arrangement. More specifically the current measurementapparatus may comprise a switch arrangement which is operative to selectan end of the measurement arrangement to which the reference inputsignal is applied. The current measurement apparatus may be configuredsuch that the signal source forms a signal source circuit path with acurrent sensor comprised in the measurement arrangement and acquisitioncircuitry comprised in the measurement arrangement forms an acquisitioncircuit path with the current sensor, the signal source circuit path andthe acquisition circuit path being separate. Separation of the two pathsfrom each other may reduce the likelihood of parasitic componentspresent in the acquisition circuit path having an adverse effect on theoperation of the reference input signal. For example separation of thetwo paths may avoid a voltage drop being developed by the referenceinput signal across parasitic resistance in the acquisition circuit pathwhich would otherwise compromise measurement accuracy.

Alternatively or in addition the processing apparatus may be operativewhen determining the load drawn current signal to adjust for thereference input signal. For example in certain arrangements theprocessing apparatus may be operative to subtract the reference inputsignal from determined load drawn current. Where the reference inputsignal is less than a desired proportion of the load drawn currentsignal, e.g. such that the load drawn current can be measured accuratelyto within ±0.1%, there may be no need to adjust for the reference inputsignal.

The processing apparatus may be configured to extract from the outputsignal from the measurement arrangement at least one of: the referenceoutput signal; and the load output signal. The processing apparatus maybe configured to perform a frequency transform on the output signal,such as a Fast Fourier Transform (FFT) algorithm, which is operative toseparate the reference output signal and the load output signal fromeach other on the basis of their different frequencies. At least one ofthe reference input signal and the load output signal may comprise acomplex signal. The processing apparatus may therefore be operative toperform a frequency analysis to determine a fundamental frequency and atleast one harmonic frequency. Accurate determination of the transferfunction or change in transfer function of the measurement arrangementmay depend on determination of the fundamental frequency and at leastone harmonic frequency. An extent to which harmonics may need to betaken into account may depend on a profile of a signal's power spectrumin the frequency domain.

The current measurement apparatus may comprise a filter array, which isoperative to extract at least one of the reference output signal and theload output signal. The transfer function or change in transfer functionof the measurement arrangement may then be determined on the basis ofthe extracted reference output signal. Configuration of the filter arraymay depend on at least one of: a frequency profile of at least one ofthe reference input and load drawn current signals; and an accuracy towhich the filter array is to be operative. The filter array may becomprised in an analogue circuit such that, for example, filtering takesplace before analogue to digital conversion or in digital circuitry suchthat, for example, filtering takes place after analogue to digitalconversion.

At least one of the measurement arrangement and the processing apparatusmay cause distortion. For example the measurement arrangement maypresent a distortion causing complex impedance to the reference inputand load drawn current signals. Accurate determination of the load drawncurrent signal may require characterisation of a distortion causing partof the current measurement apparatus. According to one approachcalibration apparatus may be used to characterise the distortion causingpart. For example the reference output signal may be characterised atdifferent amplitudes of the load drawn signal in order to determine thechange in amplitude of the reference signal at different load signalamplitudes and to determine a correction curve. The current measurementapparatus may then be configured to transform in dependence on thecharacterisation at least one of: the reference input signal; thetransfer function for the measurement arrangement as determined by theprocessing apparatus; and the load drawn current signal as determined bythe processing apparatus. According to another approach a knownreference input signal may be applied to the measurement arrangement anda corresponding signal developed by the measurement arrangementdetermined by the current measurement apparatus. The processingapparatus may be operative to analyse the determined signal, e.g. by wayof a frequency transformation approach, in relation to the knownreference input signal to provide for characterisation of at least oneof the measurement arrangement and the processing apparatus. The currentmeasurement apparatus may then be configured to operate accordingly,e.g. by transformation of a subsequently applied reference input signalor determined transfer function. Configuration may be by way of analgorithm or a lookup table.

Under certain circumstances there may be a large difference in themagnitudes of the load output signal and the reference output signalsuch that the current measurement apparatus may not be operable, e.g. onaccount of a limited dynamic range such as of acquisition circuitry, toreadily extract the reference output signal. According to a furtherembodiment the current measurement apparatus may therefore furthercomprise second processing apparatus, which is configured tosubstantially solely acquire the reference output signal and then todetermine the transfer function or change in transfer function of themeasurement arrangement in dependence on the reference input signal andthe acquired reference output signal. The transfer function may bedetermined by the first (i.e. previously described) processing apparatussuch that the second processing apparatus comprises acquisitioncircuitry. Furthermore the second processing apparatus may be operativeto convey the determined transfer function or change in transferfunction to the first processing apparatus, e.g. for determination ofthe load drawn current signal, where the first processing apparatus hasnot determined the transfer function or change in transfer function. Thetransfer function or change in transfer function may be determined on aperiodic basis, whereby the transfer function or change is determinedless frequently than the load drawn current signal. Determination of thetransfer function or change in transfer function on a regular orirregular basis is described elsewhere herein.

The second processing apparatus may be configured to receive the outputsignal from the measurement arrangement and may comprise a filtercircuit and an acquisition circuit, the filter circuit being operativeto at least partially obstruct the load output signal and theacquisition circuit being operative on an output from the filtercircuit. In use the filter circuit may at least partially obstruct ifnot substantially remove the load output signal whereby the acquisitioncircuit may be operative with a smaller dynamic range than may berequired had the acquisition circuit been operating on both thereference output signal and the load output signal. The filter circuitmay comprise a low pass filter. A cut-off frequency of the low passfilter may be less than substantially 50 Hz, substantially 10 Hz orsubstantially 1 Hz. The reference output signal may therefore comprise acomponent or consist essentially of at least one component having afrequency of less than 50 Hz, 10 Hz or 1 Hz whereas the load outputsignal may have a frequency component of 50 Hz or more. The signalsource may be configured such that a frequency of the reference signalis lower than a frequency of the current signal. Alternatively thesignal source may be configured such that a frequency of the referenceoutput signal is higher than a frequency of the load output signal, e.g.higher than a fundamental frequency and at least one harmonic frequency.The filter circuit may comprise a high pass or band pass filter. Thefilter circuit may be configured to pass a frequency component of morethan 62.5 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz or 8 kHz. Thereference output signal may therefore comprise a component or consistessentially of at least one component having a frequency of more than62.5 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz or 8 kHz whereasthe load output signal may have a dominant frequency component of 50 or60 Hz.

According to another embodiment the current measurement apparatus mayfurther comprise an extraction circuit and a subtraction circuit. Thecurrent measurement apparatus may be configured such that the extractioncircuit is operative to receive an output signal from the measurementarrangement and to extract the reference output signal from the receivedoutput signal. The subtraction circuit may be operative to subtract theextracted reference output signal from an output from the processingapparatus to thereby leave the load output signal. The extractioncircuit may comprise a demodulator circuit, e.g. in the form of a mixeror switched capacitor circuit, the signal source may be operative toapply a reference input signal to the measurement arrangement at apredetermined frequency and the demodulator circuit may be operative atthe predetermined frequency.

The current measurement apparatus may further comprise a noise filterwhich is operative to reduce noise forming part of the output signalfrom the measurement arrangement. The noise filter may be configured toreduce impulse signals and other noise of predetermined frequency whichotherwise might be liable to give rise to inaccuracy in currentmeasurement or cause other fault conditions in the current measurementapparatus. The current measurement apparatus may be configured such thata noise filter is operative to at least one of: filter signals receivedby the processing apparatus; and filter signals from the signal source.

The current measurement apparatus may further comprise at least oneconductor electrically coupled to the load and through which the currentsignal is drawn. The measurement arrangement may be disposed proximatethe at least one conductor whereby the current signal in the conductorelectromagnetically induces an induced current signal in the measurementarrangement. Alternatively or in addition the signal source may comprisea signal source conductor which is operative to carry the referenceinput signal. The measurement arrangement may be disposed proximate thesignal source conductor whereby the reference input signal in the signalsource conductor induces an induced reference signal in the measurementarrangement. The measurement arrangement may comprise a coil disposedaround the at least one conductor. The coil may also be disposed aroundthe signal source conductor. The coil may be comprised in one of acurrent transformer and a Rogowski coil. In a form the coil may bedisposed around plural live conductors. The coil may be formed around aferrite core. In another form the coil may be disposed around at leastone live conductor and a neutral conductor. The processing apparatus maybe configured to determine a characteristic transfer function of thecurrent transformer or Rogowski coil as determined on the basis of thereference input signal and the induced reference signal. The transferfunction may include the effects of parasitics of the coil, the core,the burden resistor and the processing chain and these effects maytherefore be determined and compensated. The transfer function maycomprise at least one characteristic of frequency response, such as atleast one of phase or group delay and relative amplitude response. Acharacteristic change may, for example, be indicative of saturation of acore of the current transformer or Rogowski coil and may be used todetermine if tampering is occurring through application of an externalmagnetic field or may be used to determine other fault or operablecondition events. Where the measurement arrangement comprises a currenttransformer the number of windings in the secondary of the currenttransformer with respect to the conductor going through the currenttransformer may be less than 10:1, 100:1, 250:1, 500:1, 1000:1, 2000:1,4000:1, 8000:1, 16000:1 or 32000:1. The burden resistor of the currenttransformer maybe less than 1 k ohm, 500 ohm, 250 ohm, 125 ohm, 64 ohm,32 ohm, 16 ohm, 8 ohm, 4 ohm, 2 ohm or 1 ohm.

The current measurement apparatus may comprise a measurement assemblycomprising the measurement arrangement, the at least one conductor andthe signal source conductor. The measurement assembly may be configuredto maintain the spatial separation of at least part of the measurementarrangement, such as a coil of a current sensor, the at least oneconductor and the signal source conductor amongst each other to within apredetermined limit. Maintaining spatial separation to within apredetermined limit may provide for more accurate measurement of thecurrent signal, e.g. by reducing misalignment of magnetic fields. Thespatial separation may be maintained by the measurement arrangement, theat least one conductor and the signal source conductor being bonded inthe measurement assembly or by the measurement assembly being configuredto hold the measurement arrangement at least in part, the at least oneconductor and the signal source conductor.

The measurement arrangement may comprise at least one of a shuntresistor, a current transformer, a Rogowski coil, a Hall effect sensorand other electromagnetic current sensor. The Hall effect sensor may bein the form of a ring sensor or a split ring sensor. The Hall effectsensor may be disposed relative a conductor carrying the load drawncurrent signal. In addition the Hall effect sensor may be disposedrelative a conductor carrying the reference input signal. The load drawncurrent signal and the reference input signal may be carried by separateconductors.

The current sensor may constituted by an electrical arrangementconfigured to be operative as other than a current sensor and which isadapted to be operative as a current sensor. Alternatively or inaddition the current sensor may be constituted by a conductor formingpart of and providing electrical connectivity in and perhaps to acomponent, such as a component comprised in the current measurementapparatus. More specifically the current sensor may comprise at leastone of: a length of electrical wire; a length of electrically conductivetrack formed in or on a printed circuit board; a structure formed in anintegrated circuit; a conductor comprised in an integrated circuitpackage, such as a lead frame; a structure formed as part of aconnector; and a structure formed as part of a conductive element of aseries component such as a relay. Wires, leads and lengths of conductivetrack are normally not used in prior art approaches to accurate currentmeasurement because of the effects of electromigration and temperaturedrift. A connector or a large physical component such as a capacitor ora relay within a meter may have a lead which is used as a shunt subjectto its inaccuracy and lack of precision being capable of compensation.Use of a conductor comprised in a lead frame of an integrated circuitpackage or in a structure formed as part of the integrated circuit hasthe advantages of saving on time and cost and couples the current sensorin the form, for example, of a shunt resistor more tightly to theelectronics. As described herein the present invention addresses suchproblems and therefore provides for use of such lower qualitycomponents. Where the current sensor comprises a shunt resistorarrangement, the shunt resistor arrangement may have a resistance ofless than substantially 100 mΩ. More specifically the shunt resistorarrangement may have a resistance of less than substantially 40 mΩ 20mΩ, 10 mΩ, 5 mΩ, 1 mΩ, 500μΩ, 250μΩ, 100μΩ, 50μΩ, 25 μΩ, or 5 μΩ.Alternatively or in addition the s hunt resistor arrangement may have aresistance of more than substantially 5μΩ, 25μΩ, 50μΩ, 100μΩ, 250μΩ, 500μΩ, 1 mΩ, 5 mΩ, 10 mΩ, 20 mΩ or 40 mΩ.

Alternatively or in addition and where the load draws current from asingle phase mains electricity supply, a split phase supply (i.e. wherethere are two live conductors and one neutral conductor), a three-phaseelectricity supply or a supply with more than three phases, ameasurement arrangement may be disposed in relation to each of at leastone of: a neutral wire; and at least one live wire. More specificallythe current measurement apparatus may comprise a first measurementarrangement disposed in relation to the live wire, a second measurementarrangement disposed in relation to the neutral wire, at least onesignal source and at least one processing apparatus. The currentmeasurement apparatus may comprise: first and second measurementarrangements or first, second and third measurement arrangementsdisposed in relation to a respective live wire of split or three-phaseapparatus; at least one signal source; and at least one processingapparatus. More specifically a first signal source and a firstprocessing apparatus may be electrically coupled to the firstmeasurement arrangement and a second signal source and a secondprocessing apparatus may be electrically coupled to the secondmeasurement arrangement. In three-phase apparatus a third signal sourceand a third processing apparatus may be electrically coupled to thethird measurement arrangement. Each signal source and processingapparatus pair may be operative to determine the current signal passingthrough its electrically connected measurement arrangement. Inconfigurations having multiple phases where the return path has a commonneutral, the current measurement apparatus may be operative to estimatethe content of each of the multiple phases on the neutral. The currentmeasurement apparatus may be further operative to separate the contentof each phase and to determine which phase is contributing to a currenterror. The current measurement apparatus may be further operative suchthat this process is iterative. In addition the current measurementapparatus may be operative to perform further analysis of the loadsignal on each phase to thereby reduce crosstalk in computation betweeneach phase.

Alternatively or in addition the load drawn current signal may be atleast 0.1 Amp peak or RMS. More specifically the load drawn currentsignal may be at least 1 Amp peak or RMS, 5 Amps peak or RMS, 10 Ampspeak or RMS, 20 Amps peak or RMS, 40 Amps peak or RMS, 80 Amps peak orRMS, 100 Amps peak or RMS, 200 Amps peak or RMS or 320 Amps peak or RMS.

Alternatively or in addition and where the load drawn current signal isan alternating current, the fundamental frequency of the alternatingcurrent may be less than 500 Hz, such as a frequency of substantially 60Hz or substantially 50 Hz for domestic mains or a frequency ofsubstantially 400 Hz for mains in ships or aircraft. Alternatively or inaddition a frequency of the reference input signal may be less than 250kHz, 100 kHz, 50 kHz, 20 kHz, 10 kHz, 5 kHz, 2.5 kHz or 1.25 kHz, 625Hz, 320 Hz, 160 Hz, 80 Hz or less than the fundamental of the load drawncurrent.

The current measurement apparatus may further comprise at least onepower conversion apparatus, which is operative to draw electrical powerfrom an electricity supply and to provide electrical power to thecurrent measurement apparatus. For example the power conversionapparatus may be electrically connected to a live and a neutral wire ofa mains electricity supply. The power conversion apparatus may beconnected on a supply side of the measurement arrangement to avoidmeasuring current drawn by the power conversion apparatus and theconsumer paying for such power conversion apparatus drawn current. Thepower conversion apparatus may comprise one or more of an ac-dcconverter, a rectifier, a voltage regulator and a dc to dc converterdepending on whether power is drawn from an alternating or directcurrent source and on the amplitude and the variation in the amplitudeof the electrical supply to the power conversion apparatus. Plural powerconversation apparatus may be provided for respective plural processingapparatus and signal sources to provide for isolation amongst the pluralprocessing apparatus and signal sources.

Tampering with electricity consumption meters to avoid payment forelectricity consumption is acknowledged as a problem. Electricitysuppliers are interested in being able to detect tampering amongst otherthings, for example, to ensure billing continues with a penalty, to knowwhen action needs to be taken to remove a tamper, or to deny service byforcing the interruption of the supply. A crude commonly used approachinvolves connecting a wire across the live in and live out of the meterto form a bypass of the meter so that the load drawn currentsubstantially no longer passes through the meter and is no longermeasured and billed. In order for such tampering to work the wirenormally presents a smaller resistance to the load drawn current signalthan the shunt resistor in the meter. Providing a low resistance bypasspath causes a corresponding reduction in the level of current measuredby the consumption meter to the advantage of the consumer. For examplewhere the resistance of the bypass path is a tenth of the resistance ofthe shunt resistor the measured current is about 9% of the true current.

The present inventors have appreciated that the invention as hithertodescribed may be configured to address the problem of tampering.According to a further embodiment the processing apparatus may furthercomprise data storage and may be operative to at least one of: compare apresently determined transfer function in respect of the measurementarrangement, such as an impedance, with at least one previouslydetermined transfer function stored in the data storage; and compare apresently determined factor corresponding to a change in transferfunction in respect of the measurement arrangement with at least onepreviously determined factor stored in the data store. An unduedifference between or amongst plural transfer functions or factors maybe indicative of tampering. The processing apparatus may therefore beoperative to determine a difference between the presently determinedtransfer function or factor and at least one previously determinedtransfer function or factor. The processing apparatus may then beoperative to provide diagnostic data, e.g. with regards to whether ornot there has been tampering, in dependence on the comparison. Thediagnostic data may be processed and displayed locally, for example, byway of an LED or LCD display or may be communicated elsewhere. There maybe a resulting action such as denial of service as a result of thediagnostic data. Furthermore the processing apparatus may be operativeto compare the difference between the two transfer functions or factorswith a threshold value and to determine a tampering condition independence on the comparison. The threshold value may be set at theoutset, e.g. at the post-manufacture stage. Alternatively the thresholdvalue may take into account information about the environment of thesensor such as a level load drawn current or the ambient temperature andchange the value to be tolerant to changes. Alternatively oradditionally the threshold value may depend on an earlier determineddifference between two transfer functions or factors. More specificallya predetermined offset may be added to the earlier determineddifference. The predetermined offset may provide sufficient margin toallow for detection of a large and hence tamper indicative change in thetransfer function or factor. Adding an offset to an earlier determineddifference may allow for a slow increase in the difference betweentransfer function measurements or factor determinations over time, whichmay arise for reasons other than tampering. Where transfer functions orfactors are determined on a regular basis the temporal relationship ofdifferent determined transfer functions or factors may be known ordeterminable. Where a transfer function or a factor is determined on anirregular basis it may be desirable to record a time of determination ofa transfer function or factor. The current measurement apparatus maytherefore comprise time recording apparatus, such as a real time clock,and may be operative to record a time of determination of a transferfunction or factor in dependence on operation of the time recordingapparatus. The processing apparatus may therefore be operable todetermine a rate of change of transfer function or factor over time. Theprocessing apparatus may be operable to store data in the data storerelating to transfer functions or factors determined at a plurality ofinstances. The stored data may comprise at least one of: impedancevalue; factor value; difference between impedance values; change involtage signal; and rate of change of impedance over time. The currentmeasurement apparatus may therefore be operative to store a transferfunction or factor profile over time. The current measurement apparatusmay therefore be operative to compare information before and after adisconnect event, and/or to use other usage information to corroborate alikely tampering event. For example the current measurement apparatusmay be operative to determine if the average power consumption has alsochanged when other characteristics such as frequency or phase content ofthe load drawn current have remained the same. Additionally the currentmeasurement apparatus may be operative to store a value of the transferfunction at the moment when disconnect occurs and to compare the storedvalue with a value of the transfer function when the apparatus restarts.

The present inventors have appreciated that storage of transfer functionor factor information may be applied more widely than to tamperdetection. Electrical components are liable to degrade over time withthe rate of degradation normally increasing with the current carried. Anelectrical component, such as a resistive shunt, which carries a highlevel of current, is liable to suffer what may be an appreciable rate ofdegradation over time. In the fullness of time the electrical componentmay fail. Failure of the electrical component may be accompanied by arelatively sharp change in transfer function, e.g. impedance, or factorcorresponding to a change in transfer function. For example, before andafter a high current surge event, the current sensor may change itscharacteristics. Storage of transfer function or factor data by theprocessing apparatus as described above with regards to tamper detectionmay provide for failure detection. More specifically the processingapparatus may be operative to determine in dependence on stored transferfunction or factor data whether or not the measurement arrangement hasfailed. Failure may be preceded by a characteristic change in thetransfer function of the measurement arrangement or a change in a factorcorresponding to a change in transfer function. For example a rate ofchange of transfer function over time that exceeds a predeterminedthreshold may be indicative of impending failure. Alternatively acharacteristic transfer function or factor profile over time mayindicate impending failure. For example corrosion on the terminals ofthe current sensor might appear as a drift in one direction with timeand/or average power dissipated and may be independent of other effectssuch as ambient temperature. The processing apparatus may therefore beoperative to detect impending failure in dependence on an analysis of atleast one of a change in a transfer function, a transfer functionprofile, a change in a factor corresponding to a change in transferfunction and a factor profile.

Diagnostic information, such as information relating to tamper, fault orfailure detection may be stored locally, displayed locally, acted uponlocally or passed locally to a host or gateway in the currentmeasurement apparatus for analysis and subsequent action. Analysis andsubsequent action may, for example, include communication to a Wide AreaNetwork (WAN) or a Local Area Network (LAN) of a failure event, thegathering of operational statistics or denial of service amongst otherthings. The present inventors have appreciated the determination and useof diagnostic information to be of wider applicability than hithertodescribed. As described above, transfer function characteristics may beused to adjust a metered quantity such as the load drawn current withdiagnostic information being determined in addition. In widerapplication, the diagnostic information is determined and although aquantity is metered there is no adjustment of the metered quantity.According to a further aspect of the present invention there istherefore provided current measurement apparatus comprising: ameasurement arrangement configured to be disposed in relation to a loadwhich draws a current signal, the measurement arrangement beingoperative when so disposed to measure the load drawn current signal; asignal source operative to apply a reference input signal to themeasurement arrangement whereby an output signal from the measurementarrangement comprises a load output signal corresponding to the loaddrawn current signal and a reference output signal corresponding to thereference input signal; and processing apparatus which is operative toreceive the output signal and in dependence thereon provide diagnosticinformation. Embodiments of the further aspect of the present inventionmay comprise one or more further features of any other aspect of thepresent invention.

As explained above when the invention is used with a single shunt on alive conductor tampering involving a single bypass can be detected byimpedance change detection. There are, however, other approaches totampering with electricity consumption meters that include swapping thelive and neutral wires from an electricity supply so that currentmeasurement is present in the now neutral wire and connecting the nowneutral wire to earth so that no return current passes through themeter. It is known to provide current measurement in each of the liveand neutral wires to address such a tampering attempt. This techniquecan also detect single bypass tampering because the current in the livepath will be substantially different to the current measured in theneutral path. Nevertheless the technique cannot detect a double bypasstampering attempt where both live and neutral current measurements arebypassed. This is because the amount bypassed on both conductors couldbe largely the same. This technique is normally implemented with a shuntresistor on one conductor and a current transformer around the otherconductor. In view of the sensors having different characteristics andtransfer functions the limit for detecting tampering has to be setrelatively high; typically a tamper is detected if >8% delta isdetected. The present inventors have realised if the present inventionis applied as the current measurement apparatus on both conductors witha shunt resistor as the current sensor on both live and neutralconductors, it can not only detect the single bypass tamper and theswapped and earthed tamper but also the double tamper. This is becausethis approach involves making impedance measurements in both live andneutral conductors, whereby the bypass wire causes a noticeable changein the effective shunt impedance. By knowing the impedance change fromthe nominal impedance, an estimate can be made of the current beingbypassed versus the current being measured by the meter, so an estimatefor billing can still be provided. The current measurement apparatus maytherefore comprise: first and second measurement arrangements in arespective one of the live and neutral wires from an electricity supply;first and second signal sources operative to apply a reference inputsignal to a respective one of the first and second measurementarrangements; and first and second processing apparatus operative todetermine the load drawn current signal passing through a respective oneof the first and second measurement arrangements or a transfer functionin respect of the first and second measurement arrangements. First andsecond current sensors comprised in respective first and secondmeasurement arrangements may be of a same form. For example the firstand second current sensors may be shunt resistors. Alternatively thefirst and second current sensors may be of different form, e.g. one maybe a shunt resistor and the other may be a current transformer. Thecurrent measurement apparatus may further comprise comparison apparatusoperative to perform a comparison between at least one of determinedtransfer functions and load drawn current signals.

Tampering involving bypassing at least one of the live and neutral wiresmay be reflected in a change in measured impedance in respect of each ofthe live and neutral wires. The current measurement apparatus maytherefore be configured to determine a transfer function over time forat least one of the first and second measurement arrangements. Thecurrent measurement apparatus may be further configured to at least oneof: determine a change in at least one of the transfer functions overtime; and determine a change over time in relative transfer functionsfor the first and second measurement arrangements. The currentmeasurement apparatus may be operative to determine whether or nottampering has taken place in dependence on a determined change. A changein transfer function over time or in respect of one transfer function inrelation to another may be compared with a predetermined threshold,which may be stored in the processing apparatus, to thereby reduce thelikelihood of false detection of tampering. The current measurementapparatus may therefore comprise comparison apparatus operative toperform such a comparison. Tampering involving both the live and neutralwires may be less likely to cause a tamper indicating difference betweenthe transfer functions. Nevertheless this tampering approach may cause asudden change in transfer function, such as a sudden decrease inimpedance, in respect of each of the first and second measurementarrangements. Therefore the current measurement apparatus may beconfigured to determine a change of transfer function over time for themeasurement arrangements present in the live and neutral wires. Asdescribed elsewhere a factor corresponding to a transfer function of orchange in transfer function of the measurement arrangement may bedetermined instead of or in addition to the transfer function of themeasurement arrangement. Also the current measurement apparatus may beoperative on the factor instead of or in addition to the transferfunction during the course of tamper detection.

As described above failure of a measurement arrangement may give rise toa measurable change, such as a relatively sharp increase or decrease inmeasured impedance where the measurement arrangement comprises a shuntresistor. An impending failure of a measurement arrangement may resultin an uncharacteristic change in its transfer function, such as anincrease in rate of change in measured impedance. The currentmeasurement apparatus may therefore be configured as described above tostore previously acquired data, such as transfer function data or datareflecting a rate of change in a transfer function. The currentmeasurement apparatus may also be configured as described above forcomparison of presently determined data with previously acquired data.

In configurations of the current measurement apparatus comprising pluralmeasurement arrangements which do not inherently provide for isolationthe current measurement apparatus may further comprise at least onegalvanic isolator in a circuit path between measurement arrangements.Use of at least one galvanic isolator may maintain isolation betweenelectrical conductors, e.g. between the live and neutral conductor in asingle phase arrangement and between and amongst live conductors and theneutral conductor in a three phase or split-phase arrangement.

Determination of power consumption may require measurement of a linevoltage signal as well as a load current signal with power consumptionbeing determined in dependence on measured load current and line voltagesignals. The current measurement apparatus may therefore furthercomprise line voltage measuring apparatus, such as a resistive dividerarrangement, configured to measure a voltage between the live andneutral conductors. The current measurement apparatus may furthercomprise a multiplier arrangement operative to multiply measured voltageand current values to thereby determine instantaneous power. The currentmeasurement apparatus may further comprise a real time clock and thecurrent measurement apparatus may be operative in dependence on anoutput from the real time clock and the instantaneous power to determinethe energy used. The present invention may be operative to generaterelative phase characteristics of the load current measurement transferfunction with respect to the line voltage measurement to provide foralignment of load current and line voltage measurement values tocorrectly estimate instantaneous power and to calculate power qualitymetrics such as active and reactive power and power factor.

A power generator, such as a renewable energy generator, may be presenton the load side of the measurement arrangement. The power generator maybe operative to contribute to the power consumed by the load and therebyreduce the power drawn from the mains supply. Alternatively and in alocal power generation scenario if no power is being consumed by theload or if the power generator is generating more power than is requiredby the load the power generator may be operative to convey power to themains supply. The current measurement apparatus may be configured toprovide for bidirectional current measurement. More specifically thecurrent measurement apparatus may be configured: to acquire a voltagesignal developed across the measurement arrangement, e.g. by way of acapacitive sample-and-hold arrangement comprised in the currentmeasurement apparatus; and to determine a direction of power flow inrelation to the load and a level of power conveyed. For example aninstantaneous voltage acquired from across the measurement arrangementmay be proportional to the sum of the load drawn current, the referencecurrent and the negative of the generated current with a negative sumindicating power being conveyed to the supply.

According to the present invention a measurement arrangement, such as ashunt resistor, is used to measure current drawn by the load with themeasurement being used, for example, to determine the power consumed bythe load. As discussed above the transfer function of the measurementarrangement, e.g. impedance of the shunt resistor, is liable to have anunacceptable tolerance at time of manufacture or may drift and therebygive rise to inaccuracy in current measurement. A reference input signalis therefore applied, e.g. periodically, to the measurement arrangementto determine the transfer function of the measurement arrangement or achange in the transfer function with the determined transfer function orchange being used to maintain accuracy of current measurement.Determining the transfer function or the change in transfer functioninvolves measuring a signal developed in the measurement arrangement bythe reference input signal and the load drawn current. The referenceoutput signal (i.e. the part of the measured signal corresponding to thereference input signal) is then separated from the load output signal(i.e. the part of the measured signal corresponding to the load drawncurrent) by the means described above and the transfer function orchange in transfer function is determined from the separated referenceoutput signal. The signal to noise ratio of the reference output signalto any signal other than the reference output signal having the samecharacteristics, such as random noise or part of the load output signalhaving similar characteristics to the reference input signal, e.g. inthe frequency band of the reference output signal, may however beinsufficient to allow for separation of the reference output signal anddetermination of the transfer function or change in transfer functionwithin a sufficiently short time to enable changes in the transferfunction to be followed accurately and for accuracy of currentmeasurement to be maintained.

An approach to addressing the problem involves increasing the power ofthe reference input signal to thereby improve the ratio of the referenceoutput signal to the load output signal. However this approach mayresult in power consumption being increased to an unacceptable level. Analternative approach to addressing the problem springs from anappreciation that the load output signal may either be alreadydetermined or may be determinable. The alternative approach may involvesubtracting the load output signal from the measured signal to therebyleave the reference output signal and whatever noise there may be, suchas noise generated by the processing electronics. The currentmeasurement apparatus may therefore be configured to determine the loadoutput signal and to subtract the load output signal from a signalmeasured by the measurement arrangement to thereby provide a referenceoutput signal. Furthermore the current measurement apparatus may beconfigured to determine a transfer function or a change in the transferfunction of the measurement arrangement in dependence on the referenceoutput signal. The current measurement apparatus may be configured todetermine the load drawn current in dependence on the determinedtransfer function or to adjust the determination of load drawn currentin dependence on the change of the transfer function or on a change inthe reference input signal itself. The earlier described approaches toseparation of the reference output signal, such as the FFT approach, maybe employed in addition to the present subtraction approach to at leastone of address noise and provide for improved determination, e.g. withregards to accuracy, of the transfer function or the change in thetransfer function.

According to an embodiment the current measurement apparatus maycomprise a first measurement arrangement and a second measurementarrangement, the current measurement apparatus being configured suchthat the first measurement arrangement measures the reference inputsignal and the load drawn current and such that the second measurementarrangement measures the load drawn current absent the reference inputsignal. The current measurement apparatus may be configured such thatthe reference input signal is applied to the first measurementarrangement alone of the first and second measurement arrangements. Thecurrent measurement apparatus may be configured to acquire a signal byway of each of the first and second measurement arrangements, e.g. bysampling, analogue to digital conversion and reconstruction as describedelsewhere. The current measurement apparatus may be configured such thatat least one of the sampling, analogue to digital conversion andreconstruction is carried out by different circuits in respect of eachof the first and second measurement arrangements, such as by sampling,analogue to digital conversion and reconstruction circuitry comprised ineach of the first and second measurement arrangements.

The first and second measurement arrangements may be the substantiallythe same or different arrangements. More specifically the first andsecond measurement arrangements may be operable according to the sameelectrical principle or by different electrical principles. For examplethe first and second measurement arrangements may both comprise shuntresistors. By way of further example the first measurement arrangementmay comprise a shunt resistor and the second measurement arrangement maycomprise a current transformer, such as a current transformer disposed,e.g. in relation to the neutral conductor, for tamper detection; itbeing noted that a signal on the live conductor normally returns by wayof the neutral conductor. Thus at least a part of one of the first andsecond measurement arrangements, e.g. a current sensor, may be disposedin relation to the live conductor and at least a part e.g. a currentsensor, of the other of the first and second measurement arrangementsmay be disposed in relation to the neutral conductor. Alternatively atleast a part of the first and second measurement arrangements may bedisposed in relation to one of the live and neutral conductors. Forexample the first and second measurement arrangements may comprise firstand second shunt resistors which are disposed in series in relation toeach other in the live conductor. The former embodiment (i.e. in whichonly one of the first and second measurement arrangements is present inthe live conductor) offers advantages over the latter embodiment (i.e.in which both the first and second measurement arrangements are presentin one conductor). More specifically the former embodiment involvesmeasuring the load drawn current signal absent the reference inputsignal by way of an already present measurement arrangement whereas thelatter embodiment involves inclusion of an additional component in thesame conductor as the first measurement arrangement. Furthermore theformer embodiment may measure the load drawn current signal absent thereference input signal without an appreciable increase in powerconsumption or cost. Where at least one of the first and secondmeasurement arrangements provides for no isolation from the conductorsthe current measurement apparatus may comprise an isolator configured toprovide for isolation between the first and second measurementarrangements.

Where at least a part of the first and second measurement arrangementsare disposed in relation to one of the live and neutral conductors thefirst and second measurement arrangements may comprise discretecomponents, such as two discrete shunt resistors. Alternatively thefirst and second measurement arrangements may be combined at least inpart whilst yet operating as different components. More specifically thecurrent measurement apparatus may comprise a core component configuredsuch that the first and second measurement arrangements are comprised atleast in part in the core component. The core component may comprise adistributed operative part and at least three electrical connectionswith the electrical connections being connected to the distributedoperative part, e.g. at respective different locations on thedistributed operative part. In addition electrical connections to thefirst measurement arrangement and to the second measurement arrangementmay each be constituted by different ones of the at least threeelectrical connections. The first measurement arrangement and the secondmeasurement arrangement may therefore be constituted at least in part bya part of an electrical arrangement, such as a wire or track of anintegrated circuit or part of the package lead frame for an integratedcircuit. According to one example and where the core component is aresistive element, a first electrical connection may be connected to afirst end of the resistive element, a second electrical connection maybe connected to a second opposite end of the resistive element and athird electrical connection may be connected between the first andsecond connections, e.g. at the midpoint of the resistive element. Inthis example the electrical connections to the first measurementarrangement may be constituted by the first and third electricalconnections and the electrical connections for the second measurementarrangement may be constituted by the second and third electricalconnections. According to another example and where the core componentis a resistive element, a first electrical connection may be connectedto a first end of the resistive element, a second electrical connectionmay be connected to a second opposite end of the resistive element, athird electrical connection may be connected at a first location betweenthe first and second connections, e.g. one third of the way between thefirst and second connections, and a fourth electrical connection may beconnected at a second location between the first and second connections,e.g. two thirds of the way between the first and second connections. Inthis example the electrical connections to the second measurementarrangement may be constituted by the first and second electricalconnections and the electrical connections for the first measurementarrangement may be constituted by the third and fourth electricalconnections. By way of further example the core component may beconfigured for further connections so as to provide for a multiple pointKelvin arrangement.

According to another embodiment the current measurement apparatus maycomprise first and second signal sources, the first signal source beingoperative to apply a first reference input signal to the firstmeasurement arrangement and the second signal source being operative toapply a second reference input signal to the second measurementarrangement. The current measurement apparatus may be operative to applythe first and second reference input signals to their respectivemeasurement arrangements at substantially the same time. The first andsecond measurement arrangements may be disposed for measurement inrelation to the same conductor, such as the live conductor. The currentmeasurement apparatus may further comprise first acquisition circuitrywhich is operative to acquire a first signal from a first current sensorand second acquisition circuitry which is operative to acquire a secondsignal from a second current sensor. The first and second referenceinput signals may be different. More specifically the first and secondreference input signals may be different in respect of at least one ofphase, frequency and modulation. The current measurement apparatus maybe operative to subtract each of the first and second signals from theother to thereby provide first and second subtracted signals which eachlack the load output signal. The two subtracted signals may providebetween them for determination of further information, e.g. with regardsto a change in the transfer characteristics of the first and secondmeasurement arrangements.

According to a yet further embodiment the signal source may beconfigured to modulate the reference input signal before its applicationto the measurement arrangement. According to this embodiment thereference output signal may be extracted in dependence on operation of asingle measurement arrangement in contrast to the previous embodimentswhich use two measurement arrangements. Modulation may be by way of aform of Return To Zero (RTZ) approach or the like. Modulation may bealigned to the fundamental frequency of the load drawn signal infrequency (for example, changed at a rate that is a multiple of the loaddrawn signal frequency) and or phase (for example, aligned to the zerocrossing point of the load drawn signal or the line voltage).Additionally or alternatively modulation may comprise sub-harmonicmodulation where at least one characteristic of the reference inputsignal is different during different cycles of the load drawn currentsignal. The processing apparatus may be operative to process and ifrequired store information from different cycles of the load drawncurrent signal to extract information from the modulation. Furthermorethe processing apparatus may comprise a demodulator which is operativeto separate the signal acquired from the measurement arrangement into afirst signal comprising the reference output signal and the load outputsignal and a second signal comprising the load output signal andsubstantially lacking the reference output signal. To aid modulation anddemodulation it may be beneficial for the signal source and signalacquisition circuitry to be synchronous. In addition to the first andsecond signals having a difference in reference signal content, they maycontain largely the same spectral content as the load output signalalthough with different phase relationships because they are demodulatedthrough the same chain. After alignment and/or equalisation theprocessing apparatus may then be operative to subtract the second signalfrom the first signal to thereby obtain the reference output signal. Theprocessing apparatus may be operative to only use part of theinformation available, for example in a particular relative time orphase of the load drawn current signal. The processing apparatus may beoperative to filter or discard values that are deemed by operation ofthe processing apparatus to be outside a normal operation range, tohandle spurious noise or to handle changing load conditions. The presentembodiment offers the advantage over the previously describedembodiments of relying on one measurement arrangement. The presentembodiment therefore offers the prospect of lower power consumption andlower cost than the previously described embodiments.

The processing apparatus may be configured to subtract the load outputsignal from the measured signal to thereby extract the reference outputsignal. Thereafter the current measurement apparatus may be operative asdescribed elsewhere to determine the load drawn current signal, e.g. bydetermining a factor corresponding to a change in the transfer functionof the measurement arrangement. Subtraction of the load output signalmay degrade the signal to noise ratio because of the random noise floorof the processing electronics, e.g. by 3 dB in embodiments where thenoise contribution by the first measurement arrangement processing chainis the same as the noise contribution by the second measurementarrangement processing chain. The current measurement apparatus maytherefore be operative to subtract the load output signal when the loadoutput signal is more than a predetermined amount, such as 3 dB, abovethe noise floor. The current measurement apparatus may be operative todetermine whether or not the load output signal is more than apredetermined amount above the noise floor and to selectively subtractthe load output signal in dependence on the determination. Where thereis no subtraction of the load output signal, the current measurementapparatus may employ previously described approaches to separation ofthe reference output signal, such as the FFT approach, instead of thepresent subtraction approach. In addition the processing apparatus maybe configured to combine, by addition, signals acquired by way of thefirst and second measurement arrangements. The signals may be combinedin respect of signal energy outside the reference output signal.Combining the signals in this fashion may improve the signal to noiseratio for frequencies other than frequencies of the reference inputsignal to thereby provide for improved load drawn current measurement.The presently described embodiments involving subtraction may be appliedin configurations comprising multiple phases. More specifically thepreviously described approach of estimating the content of each of themultiple phases on the neutral may be applied in such configurations.

The processing apparatus may be configured to provide for equalisationof a first signal comprising the reference output signal and the loadoutput signal with a second signal comprising the load output signalabsent the reference output signal. Equalisation may be in respect of atleast one of amplitude and phase. For example the processing apparatusmay be configured to perform an FFT on each of the first and secondsignals and to effect equalisation in dependence on analysis of thetransformations. More specifically the processing apparatus may beoperative to identify at least one largest element in the frequencydomain. For example where the current measurement apparatus is operativeto measure mains at 50 Hz, the largest element in the frequency domainmay be found at 50 Hz. The processing apparatus may then be operative toequalise the first and second signals with respect to their power in thefrequency domain at 50 Hz. The processing apparatus may be operative toanalyse elements at least one of less than and greater than anidentified element in the frequency domain and in dependence on therelative power at such elements determine whether or not equalisation isrequired and if so an extent of equalisation required. The analysis forequalization and the resulting transformation may take into accountmagnitude and phase information of each signal. By way of furtherexample the processing apparatus may be configured to perform at leastone analysis in the time domain, such as peak detect, RMS, average andcounts over a threshold. For example the processing apparatus may beconfigured to perform a peak detect on each of the two acquired signalsand to effect equalisation in dependence on the peak detections.Equalisation may be applied in each of the embodiments described above.Where current measurement by the current measurement apparatus involvesat least one inductive component, such as a current transformer,equalisation is liable to be more involved than if current measurementis performed by way of a substantially resistive component, such as ashunt resistor.

The current measurement apparatus may be constituted such that thesignal source is always present with the measurement arrangement and theprocessing apparatus. For example the signal source, the measurementarrangement and the processing apparatus may be permanently in situ.According to another approach the current measurement apparatus may beconstituted such that the signal source is separable from themeasurement arrangement and the processing apparatus. More specificallythe signal source may be comprised in a first unit and the measurementarrangement and the processing apparatus may be comprised in a secondunit. The first unit may be configured to be portable, such as handportable. The second unit may be permanently in situ. The first unit maybe brought into use, for example by a calibration engineer, as the needarises. The current apparatus may additionally include a connection toallow one of the reference input signal, the circuit operative to setthe reference input signal or the acquisition circuit, amongst otherparts of the current measurement apparatus, to be checked against anexternal known reference to determine the installed accuracy of theapparatus. For example the current measurement apparatus may have ameans by which the reference input signal is measured by externallyattached test equipment.

Where a load draws current from a multi-phase mains electricity supplyaccording to a second aspect of the present invention there may beprovided current measuring apparatus comprising plural currentmeasurement apparatus according to the first aspect of the presentinvention, each of the plural current measurement apparatus beingconfigured to measure current in a different one of the plural livewires of the electrical supply. For example the multi-phase mainselectricity supply may be a split-phase supply, a three phase supply oreven a supply with more than three phases. Embodiments of the secondaspect of the invention may comprise one or more features of the firstaspect of the invention.

Measurement of power consumption may depend on measurement of current inthe live wires of each phase. Each current measurement apparatus maytherefore comprise line voltage measuring apparatus and a multiplierarrangement as described above, whereby each current measurementapparatus may be operative to determine an instantaneous power value foreach of the phases. The current measuring apparatus may also compriseadding apparatus which is operative to add outputs from each of theplural current measurement apparatus to thereby provide a summedinstantaneous power consumption value. The summed instantaneous powerconsumption value may be used for consumption monitoring purposes. Thecurrent measuring apparatus may further comprise a real time clock andmay be operative upon summed instantaneous power consumption values independence on an output from the real time clock to provide an energyusage value. The current measuring apparatus may further comprise atleast one galvanic isolator to maintain isolation between or amongst thesupply conductors. The number of galvanic isolators required may dependon where the adding apparatus is disposed in the current measuringapparatus. For example and where the adding apparatus is constituted asa circuit element apart from all the current measurement apparatus acircuit path between each current measurement apparatus and the addingapparatus may comprise a galvanic isolator. Alternatively and where theadding apparatus is comprised in one of the current measurementapparatus a circuit path between each of the other current measurementapparatus may comprise a galvanic isolator.

In addition where there are multiple legs of a given phase each withcurrent measurement, for example in the distribution box of a multipledwelling unit, one line voltage measurement apparatus may be used withmultiple current measurement apparatus, such that the power calculatedfor each leg is based on the common voltage measurement multiplied bythe unique leg current measurement.

The current measuring apparatus may further comprise a further currentmeasurement apparatus configured to measure current flowing in a neutralwire of the multi-phase supply. Measurement of the return currentflowing in the neutral wire may provide for tamper detection byproviding for comparison of the sum of currents flowing in the livewires with current flowing in the neutral wire. The current measuringapparatus may therefore comprise adding apparatus and comparisonapparatus, the adding apparatus being operative to add measured currentfrom the plural current measurement apparatus and the comparisonapparatus being operative to compare an output from the adding apparatuswith measured current from the further (neutral wire) currentmeasurement apparatus. Furthermore the current measuring apparatus maybe configured to make a determination with regards to tampering independence on an output from the comparison apparatus. The currentmeasuring apparatus may further comprise at least one galvanic isolatorof a form and function as described above and disposed so as to maintainisolation amongst the current measurement apparatus. Further embodimentsof the second aspect of the present invention may comprise one or morefeatures of the first aspect of the present invention.

According to a third aspect of the present invention there is provided acurrent measurement method comprising: applying a reference input signalby way of a signal source to a measurement arrangement disposed inrelation to a load which draws a current signal, the measurementarrangement being operative when so disposed to measure the load drawncurrent signal; receiving an output signal from the measurementarrangement in processing apparatus, the output signal comprising a loadoutput signal corresponding to the load drawn current signal and areference output signal corresponding to the reference input signal; andmaking a determination in processing apparatus in dependence on thereference output signal and the load output signal, the determinationbeing in respect of at least one of the load drawn current signal andelectrical power consumed by the load.

Embodiments of the third aspect of the present invention may compriseone or more features of the first or second aspect of the presentinvention.

According to a fourth aspect of the present invention there is providedelectrical apparatus comprising current measurement apparatus accordingto the first aspect of the present invention or current measuringapparatus according to the second aspect, the electrical apparatus beingconfigured such that the current measurement apparatus or currentmeasuring apparatus measures current passing through a part of theelectrical apparatus.

Alternatively or in addition the electrical apparatus may compriseelectricity generation, transmission or distribution apparatus. Theelectrical apparatus may, for example, be constituted by an electricitymeter or a distribution box with the current measurement apparatus beingoperative to measure current passing through the electricity meter ordistribution box. The current measurement apparatus may thereby providea means to measure the electricity consumed by a residence, business orelectrically powered device or generated by generation apparatus, suchas a solar panel based generator. Alternatively or in addition theelectrical apparatus may comprise electrical propulsion apparatuscomprising an electrical energy storage or generation device, such as abattery or fuel cell. The electrical propulsion apparatus may beconfigured such that the current measurement apparatus is operative toprovide for regulation, e.g. by measurement of direct current, of atleast one of: power sourced by the electrical energy storage orgeneration device; and power sunk by the electrical energy storagedevice, e.g. during charging. Safe and reliable delivery of electricalpower to electric motors at high current levels is normally required ofsuch electrical propulsion apparatus. Accurate current measurement maytherefore be required to provide for proper regulation and control andto monitor for fault conditions. Alternatively or in addition theelectrical apparatus may comprise electrical control apparatuscomprising an electrical actuator. The electrical control apparatus maybe configured such that the current measurement apparatus is operativeto measure current drawn by the electrical actuator. The electricalactuator may comprise a motor and the current metrology apparatus may becomprised in a motor controller which is operative to control the motor.Electrical control apparatus may be used in diverse fields, such asmanufacturing, commercial machinery and process control. For example theelectrical actuator may comprise a stepper motor forming part of a CNCmachine or driving a valve in a fluid control system. Alternatively theelectrical actuator may comprise a linear solenoid in an electricallycontrolled automotive transmission system. In such applications accuratemeasurement of current may provide for precision of control. Furtherembodiments of the fourth aspect of the present invention may compriseone or more features of any previous aspect of the present invention.

According to a yet further aspect of the present invention there istherefore provided current measurement apparatus comprising: ameasurement arrangement configured to be disposed in relation to a loadwhich draws a current signal, the measurement arrangement beingoperative when so disposed to measure the load drawn current signal; anda signal source operative to apply a reference input signal to themeasurement arrangement whereby an output signal from the measurementarrangement comprises a load output signal corresponding to the loaddrawn current signal and a reference output signal corresponding to thereference input signal. Embodiments of the yet further aspect of thepresent invention may comprise one or more further features of any otheraspect of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following specific description, which is given by wayof example only and with reference to the accompanying drawings, inwhich:

FIG. 1A is a block diagram representation of a first embodiment ofcurrent measurement apparatus according to the present invention havinga first form of electrical connection to a shunt resistor;

FIG. 1B is a block diagram representation of a second embodiment ofcurrent measurement apparatus having a second form of electricalconnection to the shunt resistor;

FIG. 1C illustrates current measurement apparatus applied tobidirectional metering;

FIG. 2 is a block diagram representation of a third embodiment ofcurrent measurement apparatus configured to provide for tamper and faultdetection;

FIG. 3A is a block diagram representation of a fourth embodiment ofcurrent measurement apparatus configured to measure current in each of alive and neutral wire of an electricity supply;

FIG. 3B is a block diagram representation of a fifth embodiment ofcurrent measurement apparatus configured to measure current in each of alive and neutral wire of an electricity supply;

FIG. 4 is a block diagram representation of a sixth embodiment ofcurrent measurement apparatus comprising line voltage measuringapparatus;

FIG. 5 is a block diagram representation of a seventh embodiment ofcurrent shunt measurement apparatus comprising a current transformer;

FIG. 6 is a block diagram representation of measuring apparatus for athree phase electricity supply;

FIG. 7A is a block diagram representation of current measurementapparatus with a current reference circuit;

FIG. 7B shows an example of current reference circuit used in thecircuit of FIG. 7A;

FIG. 8A shows current measurement apparatus in which the currentreference circuit is applied according to a first approach;

FIG. 8B shows current measurement apparatus in which the currentreference circuit is applied according to a second approach;

FIG. 8C shows current measurement apparatus in which the currentreference circuit is applied according to a third approach;

FIG. 9 shows current measurement apparatus configured for calibration ofthe current reference circuit;

FIG. 10 shows current measurement apparatus comprising a referencesignal reference circuit;

FIG. 11 shows current measurement apparatus according to a furtherembodiment comprising a separate reference signal extraction path;

FIG. 12 shows current measurement apparatus according to a yet furtherembodiment involving analogue demodulation;

FIG. 13 shows current measurement apparatus comprising a shunt resistorand a current transformer;

FIG. 14A shows current measurement apparatus comprising a currenttransformer;

FIG. 14B shows an alternative embodiment of current measurementapparatus comprising a current transformer;

FIG. 15 shows current measurement apparatus comprising a Rogowski coil;

FIG. 16 shows current measurement apparatus comprising a Hall probe;

FIG. 17A shows a first embodiment of current measurement apparatusinvolving subtraction of the load output signal;

FIG. 17B shows a second embodiment of current measurement apparatusinvolving subtraction of the load output signal;

FIG. 17C shows a third embodiment of current measurement apparatusinvolving subtraction of the load output signal; and

FIG. 17D shows a fourth embodiment of current measurement apparatusinvolving subtraction of the load output signal.

DESCRIPTION OF EMBODIMENTS

A first embodiment of current measurement apparatus 100 having a firstform of electrical connection to a shunt resistor is shown in FIG. 1A.The current measurement apparatus 100 forms part of an electricityconsumption meter (not shown) installed at a point of supply toresidential or business premises. A single phase mains alternatingcurrent electricity source 102 with live 104 and neutral 106 supplywires are shown in FIG. 1A. Energy consuming apparatus at theresidential or business premises is represented by a load 108. Thecurrent measurement apparatus 100 comprises a shunt resistor 110 (whichconstitutes a current sensor) in the live supply wire 104 in series withthe load 108 between the load and the electricity supply 102. The shuntresistor 110 presents a low value of resistance, such as a resistance of1 mΩ. The shunt resistor 110 is formed from a length of electrical wire,a length of conductive track on a printed circuit board, a discretecomponent, a conductor comprised in an integrated circuit package suchas a lead frame or a structure formed as part of a connector orconductive element of a series component such as a relay. As will becomeapparent from the following description the shunt resistor need not beformed to provide an accurate or stable resistance; nor need theprocessing chain for the shunt resistor be accurately characterised. Thecurrent measurement apparatus 100 further comprises a signal source 112,voltage measuring apparatus 114 and signal processing circuitry 116. Theshunt resistor 110 and the voltage measuring apparatus 114 constitute ameasurement arrangement 118 and the signal processing circuitry 116constitutes processing apparatus. Although not shown in FIG. 1A thecurrent measurement apparatus comprises a noise filter at the input ofthe voltage measuring apparatus 114 to suppress undesirable noisesignals, such as impulse signals, which might otherwise be liable todisrupt operation of the current measurement apparatus. The voltagemeasuring apparatus 114 is connected to opposing ends of the shuntresistor 110 by a first pair of wires 124. The signal source 112 iselectrically connected to opposing ends of the shunt resistor 110 by asecond pair of wires 126, which are physically connected to a respectiveone of the first pair of wires 124 at locations spaced along the firstpair of wires such that the first and second pairs of wires share aconduction path. This configuration of the two pairs of wires 124, 126is appropriate where the parasitic impedance of the shared conductionpath has no adverse effect on the operation of the reference inputsignal applied by the signal source 112 to the shunt resistor 110. Inone form the current measurement apparatus 100 is constituted such thatthe signal source 112 is always present with the rest of the currentmeasurement apparatus 100 such that the signal source and the rest ofthe current measurement apparatus 100 is permanently in situ. In anotherform the current measurement apparatus 100 is constituted such that thesignal source is comprised in a separate unit from the rest of thecurrent measurement apparatus 100, which is permanently in situ. When itis desired to provide for accurate measurement, e.g. as part of aperiodic calibration procedure, the unit comprising the signal source112 is brought into use, for example by a calibration engineer, andconnected across the shunt resistor 110 before calibration begins.Calibration is described further below.

Operation of the current measurement apparatus 100 of FIG. 1A will nowbe described. As a current signal is drawn by the load 108 through theshunt resistor 110 the signal source 112 is operative on an intermittentbasis to apply a reference input signal to the shunt resistor 110 suchthat a reference current signal passes through the shunt resistor 110.The reference input signal has a known frequency or phase profile, whichis substantially absent from the load drawn current signal. For examplethe reference input signal may consist of at least one component offrequency higher than the mains frequency, such as components of afrequency greater than 5 kHz where the mains has a dominant frequencycomponent of 50 Hz such that the reference signal frequency componentslie outside a band of the mains frequency. In certain forms of theapparatus of FIG. 1A the signal processing circuitry 116 is operative tomonitor the output from the voltage measuring apparatus 114 when noreference input signal is applied to the shunt resistor 110 and todetermine a frequency or phase profile which is substantially absentfrom the output and to control the signal source 112 such that thereference input signal comprises the determined frequency or phaseprofile. Where a presently used frequency or phase profile is determinedby the signal processing circuitry 116 to now be unsuitable, the signalprocessing circuitry 116 is operative to change from the presently usedfrequency or phase profile to a different frequency or phase profile.For example the signal processing circuitry 116 is operative to changefrom 100 Hz to 83 Hz when it is determined that the load drawn currentsignal is now contaminated with 100 Hz noise. The voltage measuringapparatus 114 is operative to acquire by way of sample-and-hold andanalogue to digital conversion circuitry a voltage signal developedacross the shunt resistor 110 by the load drawn current and referenceinput signals and to generate a corresponding acquired voltage signal122. The acquired voltage signal 122 comprises a reference output signalcorresponding to the reference input signal and a load output signalcorresponding to the load drawn current signal. The signal processingcircuitry 116 is operative to extract the reference output signal fromthe acquired voltage signal 122. Extraction is achieved by applying afrequency transformation, such as in accordance with a Fast FourierTransform (FFT) algorithm, to the acquired voltage signal 122 or byfiltering the acquired voltage signal 122 on the basis of the differentfrequency profiles of the reference output signal and the load outputsignal. An extent to which harmonics of a transformed signal need to betaken into account depends on the profile of the signal's power spectrumin the frequency domain and the accuracy to which the load drawn currentsignal is to be measured. Then the signal processing circuitry 116 isoperative to determine a transfer function, e.g. complex impedance, forthe shunt resistor 110 and the voltage measuring apparatus 114 independence on the reference input signal and the extracted referenceoutput signal. The current measurement apparatus 100 stores thedetermined transfer function. Thereafter the signal processing circuitry116 is operative to determine the load drawn current signal independence on the stored transfer function and the load output signal.More specifically the reference output signal is subtracted from theacquired voltage signal 122 to determine the load output signal.Alternatively and where the reference input signal is sufficiently lowas a proportion of the load drawn current signal, e.g. 0.02% where anaccuracy of ±0.2% is desired, the load drawn current signal isdetermined in dependence on the transfer function and the acquiredvoltage signal 122, i.e. such that there is no subtraction of thereference output signal. According to another approach the signalprocessing circuitry 116 is operative to store an initial referenceoutput signal and thereafter the current measurement apparatus 100 isoperative to determine a subsequent reference output signal. If thetransfer function of the shunt resistor 110 and the voltage measuringapparatus has changed the subsequent reference output signal will havechanged in a corresponding fashion. The signal processing circuitry 116is therefore operative to determine a difference between the initial andsubsequent reference output signals and to determine a factor independence on the difference. The signal processing circuitry 116thereafter applies the factor to the load output signal to compensatefor the change in the transfer function of the shunt resistor.

The signal source 112 is operative to apply the reference input signalto the shunt resistor 110 on an intermittent basis to maintain a desiredaccuracy of measurement. Application of the reference input signal maybe regular, e.g. once per hour, day or month, where the transferfunction has an anticipated rate of drift such as might be caused by theeffects of electromigration on the shunt resistor. Alternatively or inaddition the application of the reference input signal is irregular andin dependence on a change which is liable to cause drift, such as mightbe caused by a change in temperature. Configuration of the currentmeasurement apparatus 100 to address the effects of temperature drift isdescribed below. Between applications of the reference input signal thevoltage measuring apparatus 114 is operative to determine the load drawncurrent signal in dependence on the transfer function or factor and theacquired voltage signal 122, which corresponds to the load drawn currentsignal in view of the lack of application of the reference input signalto the shunt resistor 110. In forms of the present embodiment the signalsource 112 is configured such that the reference input signal comprisesat least one of: a frequency that changes over time, e.g. changesprogressively between a first value and a second value; differentfrequencies at any one time; and frequency components which are out ofphase with each other. Having frequency components which are out ofphase is advantageous: with regards to the ease with which the referenceoutput signal is extracted from the acquired voltage signal 122; andwhere addressing a frequency dependency of the shunt resistor 110. Wherea frequency dependency of the shunt resistor 110, such as selfinductance, is being addressed, a reference input signal comprising atleast one of a frequency that changes over time and differentfrequencies at any one time is applied to the shunt resistor 110. Thesignal processing circuitry 116 is then operative to determine and storea transfer function of the shunt resistor 110 and the voltage measuringapparatus over a range of frequencies in dependence on the referenceinput signal comprising at least one of: a frequency that changes overtime; and different frequencies at any one time. These steps are carriedout during a calibration phase. Alternatively or in addition these stepsare carried out on a periodic basis during operation of the currentmeasurement apparatus. The signal source is also configured to changethe amplitude of the reference input signal. Changing the amplitude ofthe reference input signal is advantageous where the load drawn currentsignal changes between large and small values to achieve a compromisebetween measurement accuracy and power consumption.

As is described above the transfer function or a factor based on thetransfer function and thereafter the load drawn current signal aredetermined to a desired accuracy by relying on the reference inputsignal. The present invention is operative to characterise themeasurement arrangement (i.e. the shunt resistor 110 and the voltagemeasuring apparatus 114 as indicated by the dashed box around thesecomponents in FIGS. 1A and 1B) by way of the transfer function or thefactor. Having determined the load drawn current signal the currentmeasurement apparatus 100 is operative to multiply the instantaneoussupply current and line voltage to determine the instantaneous powerconsumption, which is then integrated against time to provide the energyused. The measurement of line voltage is described further below.Different approaches to relying on the reference input signal to providefor measurement to a desired accuracy are described below.

A second embodiment of current measurement apparatus 200 having a secondform of electrical connection to the shunt resistor is shown in FIG. 1B.Features in common with the current measurement apparatus 100 of FIG. 1Aare indicated by the same reference numerals and the reader's attentionis directed to the description provided above with reference to FIG. 1Afor a description of the form and function of such common features. Afirst pair of wires 124 from the voltage measuring apparatus 114 is asper FIG. 1A. A second pair of wires 128 from the signal source 112establishes an electrical connection by direct connection to opposingends of the shunt resistor 110 whereby the first and second pairs ofwires constitute separate conduction paths between the shunt resistor110 and each of the voltage measuring apparatus 114 and the signalsource 112. The arrangement of FIG. 1B is appropriate where theconduction path between the resistor shunt 110 and the voltage measuringapparatus 114 has parasitic impedance that would have a contrary effecton operation of the reference input signal. For example if a parasiticresistance of the conduction path between the resistor shunt 110 and thevoltage measuring apparatus 114 were sufficiently high in relation tothe resistance of the shunt resistor the transfer function may beinaccurately determined with there being a consequential adverse effecton accuracy of current measurement.

The current measurement apparatus 100, 200 of FIGS. 1A and 1B can beused in applications other than the measurement of current in mainselectricity consumption meters. According to such other applications theshunt resistance 110 is present in a current carrying wire in seriesbetween a first node and a second node, with one of components 102 and108 representing a voltage source, such as an electricity generator orenergy storage device, and the other of components 102 and 108representing an electrical load. Whichever of the components 102 and 108represents the voltage source is immaterial to the capability of thecurrent measurement apparatus to measure current passing through theshunt resistor 110 in either direction; this bidirectional meteringcapability is described further below with reference to FIG. 1C. In oneapplication component 102 represents a dc power source and component 108represents a load. According to this application the first currentsignal is a dc signal and the signal source 112 is configured such thatthe reference input signal applied to the shunt resistor 110 is apulsed, modulated or alternating signal. In a second applicationcomponent 102 represents an ac power source and component 108 representsa load. According to this second application the first current signal isan ac signal and the signal source 112 is configured such that thereference input signal applied to the shunt resistor 110 issubstantially a dc signal. In each of the first and second applicationsthe reference input signal has a different characteristic to the loaddrawn current signal which enables the signal processing circuitry 116to extract the reference output signal whether the extraction isachieved by way of a filtering approach or a frequency transformationapproach. Measurement of current according to these applicationsprovides for one or more of several purposes, such as regulation ormonitoring, in diverse apparatus, such as energy generation,transmission or distribution apparatus, renewable energy generators,electrical propulsion apparatus and control apparatus.

FIG. 1C illustrates bidirectional metering in block diagram form.Components of the current measurement apparatus 220 of FIG. 1C in commonwith FIG. 1A are designated with like reference numbers and the reader'sattention is directed to the description provided above with referenceto FIG. 1A for a description of such common components. The currentmeasurement apparatus of FIG. 1C further comprises a generator 230, e.g.a renewable energy source such as an array of solar panels, which islocated at the consumer's premises. When the generator 230 is operativeto generate electricity, the load 108 draws less electricity from themains electricity supply 102. In such circumstances the current passingthrough the shunt resistor 110 is the sum of the load drawn current, thereference input current and the negative of the current from thegenerator. At the time of acquisition of a voltage signal developedacross the shunt resistor 110 the acquired sample is proportional to thesum of the currents. The demand from the load 108 may cease or drop tothe extent that the generator 230 is generating electricity that issurplus to the load's requirements. Here the mains electricity supply isconfigured to receive the surplus electricity for onward transmission tothe grid. Therefore the sum of the currents is negative with the currentmeasurement apparatus 220 being operative to measure the surpluselectricity received by the mains electricity supply 102.

A third embodiment of current measurement apparatus 300 configured toprovide for at least one of tamper and fault detection is shown in FIG.2. Features in common with the current measurement apparatus 100 of FIG.1A are indicated by the same reference numerals and the reader'sattention is directed to the description provided above with referenceto FIG. 1A for a description of the form and function of such commonfeatures. Features particular to the third embodiment will now beidentified and the function of the third embodiment with respect tothese particular features will be described subsequently. The currentmeasurement apparatus 300 of FIG. 2 further comprises a CentralProcessing Unit (CPU) 302 and a data store 304, such as volatile or nonvolatile electronic memory. The CPU 302 and data store 304 are of wellknown form. During the course of operation of the current measurementapparatus 300 the data store 304 is operative to store each transferfunction or factor determined following operation of the signal source112; each transfer function or factor is stored with a time and datestamp provided by a real time clock (not shown). The CPU 302 isoperative to analyse stored transfer function or factor data todetermine a rate of change of the transfer function or factor with timeand to determine a transfer function or factor profile over time. Atamper wire 306 is shown connected across the shunt resistor 110. Thetamper wire 306 has been installed across the live in and live out of anelectricity consumption meter by an electricity consumer to causeunder-recording of current consumption to the consumer's financialadvantage. The tamper wire has a distributed resistance, which isrepresented in FIG. 2 by an equivalent resistance 308. The distributedresistance is lower than the shunt resistance. The distributedresistance of the tamper wire 306 therefore lowers the resistance seenand therefore determined by the voltage measuring apparatus 114. A lowerresistance develops a smaller potential drop which when divided inaccordance with Ohm's law by the nominal impedance of the shunt resistor110 provides a lower measured current. Determination of the actualimpedance by application of the reference input signal, as describedabove, addresses the effect of the tamper on measurement accuracy.Furthermore analysis of the stored impedance values provides for tamperdetection. More specifically a sudden, large change in measuredimpedance is indicative of tampering. Such a change in impedance isdetermined by comparing a currently acquired impedance value with anearlier acquired impedance value and if the difference is larger than athreshold value tampering is indicated. The threshold value is set totake account of impedance change for other reasons, e.g. degeneration ofthe shunt resistor over time or temperature drift. In addition the CPU302 is operative to compare impedance values acquired before and after adisconnect event and to analyse other usage information to corroborate alikely tampering event. One example of such analysis involvesdetermining whether or not the measured average power consumption(determined as described below on the basis of line voltage measurement)has also changed when other characteristics such as the frequency orphase content of the load drawn current have remained substantiallyunchanged. Analysis of the stored impedance values also provides forfault detection. Failure of the shunt resistor for reasons other thantampering may cause a relatively sharp increase or decrease in impedancewhich is reflected in determined impedance values. For example and wherethe like of a current transformer is used as the current sensor thecharacteristics of the current transformer may change after a highcurrent surge event. Impending failure of the shunt resistor may bereflected in a characteristic impedance profile over time, such as aprogressive increase in a rate of increase or decrease in impedance. Forexample corrosion on the terminals of the shunt resistor 110 may giverise to a drift in one direction over time and/or average powerdissipated with the drift being independent of other effects such asambient temperature. The CPU 302 is operative to analyse such impedancedata and make a determination with regards to failure or impendingfailure on the basis of the analysis.

In an un-illustrated embodiment which is based on the apparatus of FIG.2 there is no use made of the transfer function in improving uponmeasurement accuracy. Instead a metered quantity, such as current orpower consumption, is determined in accordance with the known approachand without taking a change in the transfer function into account and acharacteristic of the transfer function, such as an extent of change, isused to provide diagnostic information which includes tamper indication,fault indication and the like. Such diagnostic information is displayedlocally and communicated from the apparatus to a remote location such asby way of a LAN or WAN.

Current measurement apparatus 400 according to a fourth embodiment whichis configured to measure current in each of a live and neutral wire ofan electricity supply is shown in FIG. 3A. The current measurementapparatus 400 comprises a first shunt resistor 402 in the live wire 404between an ac power source 406 and a load 408 and a second shuntresistor 410 in the neutral wire 412 between the ac power source 406 andthe load 408. The current measurement apparatus 400 further comprises afirst unit 414 and second unit 416. The first unit 414 comprises a firstsignal source 418, first voltage measuring apparatus 420 and firstsignal processing circuitry 422. The second unit 416 comprises a secondsignal source, second voltage measuring apparatus and second signalprocessing circuitry. The signal source 418, voltage measuring apparatus420 and signal processing circuitry 422 of each of the first and secondunits 414, 416 are configured and operative to determine the load drawncurrent signals drawn through a respective one of the live and neutralwires 404, 412. The form and function of each of the first and secondunits 414, 416 is the same as the current measurement apparatusdescribed above with reference to FIG. 1A. The current measurementapparatus 400 of FIG. 3A further comprises a first galvanic isolator432, a second galvanic isolator 434, a comparison circuit 436, a firstpower supply apparatus 438 and a second power supply apparatus 440. Thefirst galvanic isolator 432 is present in a signal path between thefirst unit 414 and the comparison circuit 436 and the second galvanicisolator 434 is present in a signal path between the second unit 416 andthe comparison circuit 436, whereby isolation between the live andneutral wires 404, 412 and between the first and second units 414, 416is maintained. The first and second power supply apparatus 438, 440 areeach configured to provide power to a respective one of the first andsecond units 414, 416. The first and second power supply apparatus 438,440 draw power from the live and neutral wires 404, 412 on the ac powersupply 406 side of the shunt resistors 402, 410 so as to avoid thecurrent measurement apparatus measuring current drawn by the first andsecond power supply apparatus 438, 440. In accordance with known designpractice, the first and second power supply apparatus 438, 440 providesfor ac-dc conversion, rectification, regulation and whatever dc to dcconversion might be required by the first and second units 414, 416.

The operation of the current measurement apparatus 400 of FIG. 3A willnow be described. The current measurement apparatus 400 is capable ofdetecting a tamper event involving a single bypass as described abovewith reference to FIG. 2, a tamper event involving swapping the live andneutral wires from the electricity supply and a tamper event involving adouble bypass where both live and neutral current measurements arebypassed. Each of the first and second units 414, 416 is operative todetermine a transfer function of a respective one of the first andsecond shunt resistors 402, 410 and their processing chains and arespective load drawn current signal passing through the live andneutral wires 404, 412. Operation of each of the first and second units414, 416 in this respect is described further above with reference toFIG. 1A such that a change in transfer function of one or other of thefirst and second shunt resistors 402, 410 and their processing chains isindicative of single bypass tamper event. A tamper event involvingswapping the live and neutral wires does not avoid current measurementbecause current measurement is present in both the live and neutralwires. The current measurement apparatus 400 is further configured suchthat the comparison circuit 436 is operative to compare the determinedload drawn current signals or transfer functions. A difference betweenthe two load drawn current signals above a threshold value or acharacteristic change in the transfer functions indicates that one ofthe live and neutral wires 404, 412 has been tampered with. The currentmeasurement apparatus is also operative to analyse the transferfunctions determined for the first and second shunt resistors 402, 410and their processing chains. This approach is advantageous because thetwo amounts of current bypassed in a double bypass tamper could be muchthe same thereby rendering ineffective the known approach of detecting atamper on the basis of a difference between the live and neutral currentsignals. Analysis of the transfer function in respect of each of thefirst and second shunt resistors 402, 410 comprises determining a rateof change over time to detect a sudden change of transfer function thatis indicative of tampering. A sudden change of transfer function inrespect of both the first and second shunt resistors 402, 410 isindicative of tampering with both live and neutral wires 404, 412.Despite there being tampering an estimate of power consumption cannevertheless still be determined. More specifically the tamper causedchange in impedance from the nominal impedance is applied to themeasured load current to provide an estimated actual load current whichwith measured line voltage provides estimated power consumption. Thecurrent measurement apparatus is also operative to analyse transferfunction data for the purpose of fault detection. As further describedabove fault detection involves detecting a relatively sharp change intransfer function. Impending failure of the shunt resistor is detectedby determining a characteristic transfer function profile over time,such as a progressive increase in a rate of increase or decrease inimpedance.

A fifth embodiment of current measurement apparatus 500 configured tomeasure current in a live and neutral wire of an electricity supply isshown in FIG. 3B. Features in common with the current measurementapparatus 400 of FIG. 3A are indicated by the same reference numeralsand the reader's attention is directed to the description provided abovewith reference to FIG. 3A for a description of the form and function ofsuch common features. Features particular to the embodiment of FIG. 3Bwill now be identified and their operation described. The embodiment ofFIG. 3B comprises a comparison circuit 502 and a galvanic isolator 504.The comparison circuit 502 forms part of the first unit 414, e.g. thecomparison circuit 502 is formed on the same integrated circuit withthere being a conduction path between the circuitry of the first unitand the comparison circuit. The galvanic isolator 504 is present in asignal path between the second unit 416 and the comparison circuit 502to thereby maintain isolation between the first and second units 414,416. Otherwise the form and function of the fifth embodiment is the sameas the fourth embodiment. The configuration of the fifth embodiment maybe changed such that the second unit 416 lacks signal processingcircuitry and the outputs from the voltage measuring apparatus of eachof the first and second units 414, 416 are received in and processed bythe signal processing circuitry 422 of the first unit 416. In thisconfiguration a galvanic isolator is present in the signal path betweenthe voltage measuring apparatus of the second unit 416 and the signalprocessing circuitry 422 of the first unit 416 to thereby maintainisolation between the first and second units 414, 416.

A sixth embodiment of current measurement apparatus 600 comprising linevoltage measuring apparatus is shown in FIG. 4. Features in common withthe current measurement apparatus of FIG. 1A are indicated by the samereference numerals and the reader's attention is directed to thedescription provided above with reference to FIG. 1A for a descriptionof the form and function of such common features. Features particular tothe embodiment of FIG. 4 will now be identified and described. Thecurrent measurement apparatus 600 further comprises line voltagemeasuring apparatus 602, computational circuitry 604 and a real timeclock 606. The line voltage measuring apparatus 602 is a knownarrangement, such as a resistive divider, which is configured to measurevoltage across the live and neutral conductors 104, 106. Thecomputational circuitry 604 is constituted by a microprocessor or thelike and is operative to receive line voltage measurements from thevoltage measuring apparatus 602 and current measurements from the signalprocessing circuitry 116. The computational circuitry 604 is operativeto determine instantaneous power by multiplication of voltage andcurrent measurements. Energy used is determined by integratinginstantaneous power consumption over time in dependence on an outputfrom the real time clock 606. The computational circuitry 604 is furtheroperative to generate relative phase characteristics of the load currentmeasurement transfer function with respect to the line voltagemeasurement to provide for alignment of load current and line voltagemeasurement values to correctly estimate instantaneous power and tocalculate power quality metrics such as active and reactive power andpower factor.

A seventh embodiment of current measurement apparatus 700 comprising acurrent transformer is shown in FIG. 5. A description of how the CT iscalibrated so that the CT can be used for the purpose of accuratemeasurement is provided below. Features in common with the currentmeasurement apparatus of FIG. 1A are indicated by the same referencenumerals and the reader's attention is directed to the descriptionprovided above with reference to FIG. 1A for a description of the formand function of such common features. Features particular to theembodiment of FIG. 5 will now be identified and described. The currentmeasurement apparatus 700 further comprises a current transformer 702,second voltage measuring apparatus 704, a comparison circuit 706 andpower supply apparatus 708. A burden resistor 710 is electricallyconnected in parallel across the current transformer 702. The currenttransformer 702 is operative to measure current flowing in the neutralwire 106, with current induced in the current transformer 702 beingdeveloped across the burden resistor 710. The second voltage measuringapparatus 704 is operative acquire the voltage developed across theburden resistor 710 and to convert the acquired voltage into digitalform, e.g. by analogue to digital conversion in accordance with knownpractice. The shunt resistor 110 present in the live wire 104 providesfor accurate current measurement for power consumption determinationpurposes as is described above. The current transformer 702, on theother hand, provides for lower accuracy of current measurement withcurrent being measured to within ±6% to ±10% of the actual current. Thecomparison circuit 706 is operative to compare the currents measured bythe shunt resistor 110 and the current transformer 702 to determine ifthere is a sufficiently significant difference between the measuredcurrents. As described above a significant difference between currentsflowing in the live and neutral wires is indicative of tampering. Inview of the normally significant difference in current level caused bytampering, current flowing in the neutral wire can be measured to loweraccuracy. The power supply apparatus 708 is configured and is operativeas described above to provide electrical power to the currentmeasurement apparatus 700. In view of the inherently isolating nature ofthe current transformer 702 there is no need to provide for galvanicisolation between the second voltage measuring apparatus 704 and thecomparison circuit 706.

Measuring apparatus 800 for a three phase electricity supply is shown inFIG. 6. A load 802 draws current from a three phase electricity supply804 by way of first to third live wires 806, 808, 810 and a neutral wire812. First to third shunt resistors 814, 816, 818 are present in arespective one of the first to third live wires 806, 808, 810. First tothird units 820, 822, 824 measure current and line voltage in or on arespective one of the first to third live wires 806, 808, 810 asdescribed above with reference to FIGS. 3A and 4. More specifically eachunit comprises a signal source 826, voltage measuring apparatus 828 andsignal processing circuitry 830, which are operative as described abovewith reference to FIG. 1A. Each unit further comprises line voltagemeasuring apparatus 832, which is operative to measure the voltagebetween a live wire and neutral as described above with reference toFIG. 4, and multiplication circuitry 834, which is operative to multiplymeasured current and voltage to determine power. The measuring apparatus800 further comprises first to third power supply apparatus 836, 838,840, which are of a form and function as described above and which areoperative to provide electrical power to a respective one of the firstto third units 820, 822, 824. In addition the measuring apparatus 800comprises first to third galvanic isolators 842, 844, 846 and addercircuitry 848. The first to third galvanic isolators 842, 844, 846 arepresent in a respective one of the three signal paths between each ofthe first to third units 820, 822, 824 and the adder circuitry 848 andthereby maintain isolation between and amongst the first to third units.The adder circuitry 848 is operative to receive an output from themultiplication circuitry 834 of each of the first to third units 820,822, 824 to add the outputs and thereby determine power for all threephases. Although not shown in FIG. 6, in a form of the measuringapparatus a fourth shunt resistor is provided in the neutral wire 812and the apparatus further comprises further current measurementapparatus of a form already described. The measuring apparatus isfurther configured to compare current measured for the neutral wire withthe sum of the currents measured in the three live wires to provide fortamper detection. The measuring apparatus 800 of FIG. 6 may be otherwiseconfigured to provide for detection of tampering and faults as describedabove. The configuration shown in FIG. 6 is applied to a split phasearrangement by dispensing with one of the first to third shunt resistors814, 816, 818 and its associated circuitry whereby current and linevoltage is measured in two phases only. The configuration shown in FIG.6 is applied to arrangements having four or more phases by providing arespective number of shunt resistors and associated circuitry.

As described above accuracy of measurement of the transfer function ofthe shunt resistor and its processing chain relies on the referenceinput signal provided by the signal source. Therefore and according tocertain embodiments of the present invention an accurately known andstable current reference is provided. This approach is illustrated inFIG. 7A, which shows in block diagram form current measurement apparatus900 with a current reference. Features in common with the currentmeasurement apparatus of FIG. 1A are indicated by the same referencenumerals. Further to the common features the current measurementapparatus 900 comprises a current reference circuit 902 (whichconstitutes a signal source reference circuit), which is operative toset the reference input signal provided by the signal source 112. Anexample of a current reference circuit 902 is shown in block diagramform in FIG. 7B. The example of FIG. 7B is a voltage controlled currentsource 920 comprising a current mirror 922, which is driven by a biasvoltage provided by an amplifier 924 which is in turn driven by anoutput from a silicon bandgap reference 926. A current in a first leg ofthe current mirror 922 is set by a reference resistor 928 of accuratelyknown resistance and having required stability characteristics. Thecurrent in the second leg of the current mirror is provided to thesignal source 112 of FIG. 7A. According to a particular approach thecurrent measurement apparatus is constituted such that the signal source112, the voltage measuring apparatus 114, the signal processingcircuitry 116 and all component parts of the current reference circuit902 with the exception of the reference resistor 928 are formed in anintegrated circuit. The reference resistor 928 is a precision resistorexternal to the integrated circuit.

Application of the current reference circuit according to a firstapproach will now be described with reference to FIG. 8A. FIG. 8A showscurrent measurement apparatus 940 comprising components in common withFIG. 1A. The reader's attention is therefore directed to the descriptionprovided above with reference to FIG. 1A for a description of suchcommon components. The current measurement apparatus 940 of FIG. 8Afurther comprises a voltage controlled current source 942 and a bandgapreference 944, which are operative as described above with reference toFIGS. 7A and 7B. Furthermore the current measurement apparatus 940comprises a variable reference resistor 946, a trimmer 948 (whichconstitutes a reference adjustment arrangement) and One TimeProgrammable (OTP) memory 950. In addition the current measurementapparatus 940 comprises a temperature sensor 952 and a lookup table 954.According to the present approach the resistance of the variablereference resistor 946 is to be set to a predetermined value so that thereference input signal is of a predetermined current level. The variablereference resistor 946 is formed as part of an integrated circuit withthe other components of the current measurement apparatus. During acalibration procedure an accurately known and stable calibrationresistor (not shown) is electrically connected to the currentmeasurement apparatus in place of the shunt resistor 110. In dependenceon accurate measurement of the voltage signal across the calibrationresistor the actual reference input signal is determined whereby theadjustment to the variable reference resistor 946 which is required toset the reference input signal to its desired level can be determined.The required adjustment is effected by storing adjustment data in theOTP memory 950 with the trimmer 948 being operative in dependence on thestored adjustment data to set the variable reference resistor 946 suchthat it has the required resistance. The calibration procedure alsoinvolves changing the temperature of the current measurement apparatus940 over an entire operational temperature range, such as −20 to 85degrees Celsius, and measuring the reference input signal at pluraltemperatures, e.g. 1 degree steps, to thereby form a temperaturebehaviour profile for the reference input signal over the entireoperational temperature range. The temperature profile is then storedthe lookup table 954. In use the signal processing circuitry 116 isoperative in dependence on a temperature measured by the temperaturesensor 952 to determine a temperature compensation factor from thelookup table 954 and to change the determined transfer function or themeasured load output signal in dependence on the compensation factor. Incertain forms of the embodiment the calibration procedure addressesdistortion caused by the shunt resistor and/or its processing chain.Distortion is addressed by applying known signals to the componentsubject to characterisation, measuring the output signals anddetermining a transfer function for the distortion based on the appliedand output signals. For example known current signals are applied to theshunt resistor 110, the voltage signals developed across the shuntresistor are acquired and the transfer function determined accordingly.The current measurement apparatus is then configured, e.g. by way of alookup table, on the basis of the determined transformation to transformthe measured load output signal.

Application of the current reference circuit according to a secondapproach will now be described with reference to FIG. 8B. FIG. 8B showscurrent measurement apparatus 960 comprising components in common withFIG. 1A. The reader's attention is therefore directed to the descriptionprovided above with reference to FIG. 1A for a description of suchcommon components. The current measurement apparatus 960 of FIG. 8Bfurther comprises a voltage controlled current source 962 and a bandgapreference 964, which are operative as described above with reference toFIGS. 7A and 7B. Furthermore the current measurement apparatus 960comprises a fixed reference resistor 966, One Time Programmable (OTP)memory 968, a temperature sensor 970 and a lookup table 972. The fixedreference resistor 966 is formed as part of an integrated circuit withthe other components of the current measurement apparatus. According tothe present approach the reference output signal level is measured andthe current measurement apparatus configured to take account of themeasured reference output signal. More specifically and during acalibration procedure an accurately known and stable calibrationresistor (not shown) is electrically connected to the currentmeasurement apparatus in place of the shunt resistor 110. The referenceoutput signal is determined in dependence on accurate measurement of thevoltage signal across the calibration resistor. Then the referenceoutput signal is stored in the OTP memory 968. During normal use thesignal processing circuitry 116 is operative to determine the transferfunction of the shunt resistor 110 and its processing chain independence on the presently measured reference output signal and thereference output signal stored in the OTP memory 968. As described abovewith reference to FIG. 8A the calibration procedure also involvesforming a temperature behaviour profile for the reference input signalover an entire operational temperature range and configuring the lookuptable 972 accordingly. In use the signal processing circuitry 116 isoperative in dependence on an output from the temperature sensor 970 andthe content of the lookup table 972 to compensate for temperature driftas described above with reference to FIG. 8A.

Application of the current reference circuit according to a thirdapproach will now be described with reference to FIG. 8C. FIG. 8C showscurrent measurement apparatus 980 comprising components in common withFIGS. 1A and 8B. The reader's attention is therefore directed to thedescription provided above with reference to FIGS. 1A and 8B for adescription of such common components. A component particular to thecircuit of FIG. 8C will now be described. Instead of the calibration andcompensation components of the circuit of FIG. 8B (e.g. the fixedreference resistor 966, One Time Programmable (OTP) memory 968,temperature sensor 970 and lookup table 972) the circuit of FIG. 8Ccomprises a temperature stabilised precision resistor 982 (whichconstitutes a signal source reference circuit). The rest of thecomponents of the circuit of FIG. 8C with the exception of the shuntresistor 110 are constituted as a printed circuit board arrangement,multi-chip module, integrated circuit or the like and the precisionresistor 982 is an external component. The precision resistor 982 of thecircuit of FIG. 8C is operative like the fixed reference resistor 966 ofthe circuit of FIG. 8B to set the reference input signal. However thecircuit of FIG. 8B is operative by way of the calibration andcompensation components to adjust the reference input signal provided bythe fixed reference resistor 966 whereas the precision resistor 982 ofthe circuit of FIG. 8C is selected such that it determines the referenceinput signal of itself.

Current measurement apparatus 1000 configured for calibration of thecurrent reference circuit is shown in FIG. 9. Components of theapparatus of FIG. 9 in common with FIGS. 1A and 8B are designated withlike reference numbers and the reader's attention is directed to thedescription provided above with reference to FIGS. 1A and 8B for adescription of such common components. The current measurement apparatusfurther comprises a primary reference resistor 1002 (which constitutes afirst signal source reference circuit), a secondary reference resistor1004 (which constitutes a second signal source reference circuit) anddata memory 1006. The current measurement apparatus also comprises aprimary switch 1008 and a secondary switch 1010. The primary andsecondary reference resistors 1002, 1004 are formed on a printed circuitboard, in the lead frame of an integrated circuit package or in anintegrated circuit such that they are of accurately matched resistance.During ordinary use the primary switch 1008 is closed such that thereference input signal is set by the primary reference resistor 1002.The primary reference resistor 1002 is liable to degrade and itsresistance therefore liable to drift with there being a consequentialloss of accuracy of current measurement. Once every predetermined numberof cycles of current measurement, e.g. once per day or week, the currentmeasurement apparatus is operative to open the primary switch 1008 andclose the secondary switch 1010 whereby the reference input signal isset by the secondary reference resistor 1004. Then the primary switch1008 closes and the secondary switch 1010 opens whereby normal operationresumes. The secondary reference resistor 1004 carries a much lowerlevel of current over time than the primary reference resistor 1002 andis therefore much less liable to degradation and drift. The currentmeasurement apparatus 1000 is operative to determine an extent of driftin resistance of the primary reference resistor 1002 from the resistanceof the secondary reference resistor 1004 in dependence on the differencebetween the second input signal acquired when the primary referenceresistor 1002 is operative and the second input signal acquired when thesecondary reference resistor 1004 is operative. Drift data to compensatefor the extent of drift is stored in the data memory 1006. During normaluse the current measurement apparatus 1000 is operative to determine theload drawn current signal on the basis of the stored drift data.

Current measurement apparatus 1020 comprising a reference signalreference circuit is shown in FIG. 10. Components of the apparatus ofFIG. 10 in common with FIGS. 1A and 8B are designated with likereference numbers and the reader's attention is directed to thedescription provided above with reference to FIGS. 1A and 8B for adescription of such common components. The current measurement apparatus1020 further comprises a reference signal resistor 1022, a first pair ofswitches 1024 and a second pair of switches 1026. The reference signalresistor 1022 is of accurately known resistance and is of goodtemperature stability. The current measurement apparatus 1020 alsocomprises a current reference 1028 and data memory 1030. During use thefirst pair of switches 1024 is normally closed and the second pair ofswitches 1026 is normally open whereby the signal source 112 and thevoltage measuring apparatus 114 are connected across the shunt resistor110. Once every predetermined number of cycles of acquisition of voltagesignal from the shunt resistor, e.g. once every thousand cycles, thecurrent measurement apparatus is operative to open the first pair ofswitches 1024 and close the second pair of switches 1026 to therebydisconnect the signal source 112 and the voltage measuring apparatus 114from the shunt resistor and to connect the signal source 112 and thevoltage measuring apparatus 114 to the reference signal resistor 1022.Then the current measurement apparatus is operative to determine thereference output signal by acquisition of the voltage signal developedacross the reference signal resistor 1022 by the reference input signal,the resistance of the reference signal resistor 1022 being known andstored. The determined reference output signal is stored in the datamemory 1030. Normal measurement across the shunt resistor 110 is resumedby closing the first pair of switches 1024 and opening the second pairof switches 1026. During resumed normal measurement the currentmeasurement apparatus is operative to determine the impedance of theshunt resistor 110 by reference of the currently measured referenceoutput signal to the newly stored reference output current signal forthe known reference signal resistor 1022. Periodic operation of thereference signal resistor 1022 as described provides for maintenance ofaccuracy of current measurement. Thus the current reference 1028 is notrequired to provide a reference of absolute accuracy for the signalsource 112. However the current reference 1028 should be of sufficientstability between operations of the reference signal resistor 1022 tomaintain accuracy. The reference signal resistor 1022 carries a muchlower level of current than the shunt resistor 110, e.g. mA versus Amps,and is therefore much less liable to degradation than the shuntresistor. A low current level carrying resistor is more readily providedand at lower cost than a high current level carrying resistor. Thereference signal resistor 1022 is provided either as an externalprecision component or, as shown in FIG. 10, is formed in an integratedcircuit along with the other components of the current measurementapparatus. Where the reference signal resistor 1022 is formed in anintegrated circuit the current measurement apparatus further comprises atrimmer and OTP memory (not shown in FIG. 10) which are operative as aconsequence of a calibration procedure as described above with referenceto FIG. 8A to provide for adjustment of the resistance of the referencesignal resistor 1022 to a desired value. The current measurementapparatus 1020 of FIG. 10 is further operative to interpolate betweendeterminations made on each side of a measurement with the referencesignal resistor 1022 to thereby provide data that would otherwise bemissed. This approach compensates for under-measurement and is appliedwhere the under-measurement error is significant vis-a-vis the requiredaccuracy. A calibration procedure involves forming a temperaturebehaviour profile for the reference signal resistor 1022 over an entireoperational temperature range and configuring the lookup table 972accordingly. The calibration procedure is described above in more detailwith reference to FIG. 8A. In use the signal processing circuitry 116 isoperative in dependence on an output from the temperature sensor 970 andthe content of the lookup table 972 to compensate for the effects oftemperature drift as described above in more detail with reference toFIG. 8A.

Current measurement apparatus 1050 according to a further embodimentcomprising a separate reference signal extraction path is shown in FIG.11. Components of the apparatus of FIG. 11 in common with FIG. 1A aredesignated with like reference numbers and the reader's attention isdirected to the description provided above with reference to FIG. 1A fora description of such common components. The current measurementapparatus 1050 of FIG. 11 further comprises a filter 1052 and secondvoltage measuring apparatus 1054. The filter 1052 is connected acrossthe shunt resistor 110. The second voltage measuring apparatus 1054receives an output from the filter 1052. The filter 1052 is configuredto obstruct the part of the voltage signal developed across the shuntresistor 110 which corresponds to the load drawn current signal (i.e.the load output signal). According to the present embodiment thereference input signal has a frequency of less than 50 Hz whereas theload drawn current signal is at mains frequency, i.e. 50 Hz. The filter1052 is therefore operative to pass the reference output signal to thesecond voltage measuring apparatus 1054. The second voltage measuringapparatus 1054 is operative to convert the received signal to a digitalform. The signal processing circuitry 116 is operative to determine theshunt resistor impedance on the basis of the signal received from thesecond voltage measuring apparatus 1054 and the reference input signal.The signal processing circuitry 116 then determines the load drawncurrent signal on the basis of the impedance and the voltage signalacquired by the first voltage measuring apparatus 114. As describedelsewhere the signal processing circuitry 116 may or may not subtractthe reference output signal from the voltage signal acquired by thefirst voltage measuring apparatus 114 depending on whether or not thereference output signal is of sufficient magnitude to compromise theaccuracy of current measurement.

Current measurement apparatus 1060 according to a yet further embodimentinvolving analogue demodulation is shown in FIG. 12. Components of theapparatus of FIG. 12 in common with FIG. 1A are designated with likereference numbers and the reader's attention is directed to thedescription provided above with reference to FIG. 1A for a descriptionof such common components. The current measurement apparatus 1060 ofFIG. 12 further comprises an analogue demodulator 1062, which isoperative to receive a voltage signal developed across the shuntresistor 110, and a subtraction circuit 1064, which receives inputs fromthe analogue demodulator 1062 and the voltage measuring apparatus 114and provides an output to the signal processing circuitry 116. Theanalogue demodulator 1062 is of the form of a mixer, switched capacitorcircuit or the like. The signal source 112 is operative to apply aswitching reference input signal to the shunt resistor 110 at apredetermined frequency. The analogue demodulator 1062 is operative atthe same predetermined frequency to thereby extract solely the voltagedeveloped across the shunt resistor 110 by the switching reference inputsignal. The extracted voltage is passed to the subtraction circuit 1064where the extracted voltage is subtracted from the output from thevoltage measuring apparatus 114 to thereby correct for the error incurrent measurement caused by the applied switching reference inputsignal. The corrected output from the voltage measuring apparatus 114and the extracted voltage are passed to the signal processing circuitry116, which is operative to determine the impedance of the shunt resistor110 and apply a correction factor to the corrected output from thevoltage measuring apparatus 114.

Forms of the current measurement apparatus of FIG. 11 or 12 comprisedifferent forms of reference circuit described above with reference toFIGS. 7A to 10. Forms of any of the arrangements shown in FIGS. 8A to 12are configured to be operative, for example, with regards to theprovision of tamper and fault detection, the determination of power andenergy consumption and to be applied in multi-phase configurations as isdescribed above with reference to FIGS. 1A to 6.

Current measurement apparatus 1200 comprising a current transformeraccording to a first embodiment is shown in FIG. 13. The currentmeasurement apparatus 1200 of FIG. 13 comprises components in commonwith FIGS. 1A and 3B. The reader's attention is therefore directed tothe description provided above with reference to FIGS. 1A and 3B for adescription of such common components. The current measurement apparatus1200 further comprises a coil 1202 of a current transformer, which isdisposed around the live conductor 404, and a burden resistor 1204connected across the coil 1202. In one form the coil 1202 is wound on aunitary ferrite core and the live conductor 404 is passed through theaperture defined by the ferrite core. This form is appropriate: wherethe coil is fitted upon assembly and before installation whereby thelive conductor may be readily passed through the aperture; or when thelive conductor may be readily disconnected, passed through the apertureand reconnected. In another form the coil 1202 is wound on a splitferrite core, the portions of the ferrite core are separated and thelive conductor is received between the portions of ferrite core beforethe ferrite core portions are moved together again whereby the liveconductor is encircled by the ferrite core. This form is appropriatewhere the live conductor may not be readily disconnected, e.g. in aconsumer self-fitted configuration. Irrespective of the form used thelive conductor normally constitutes a unity turn primary of the currenttransformer and the coil around the ferrite constitutes an N turnsecondary of the current transformer. Another less commonly usedconfiguration involves winding the primary, i.e. live conductor, and thesecondary, i.e. the coil, on the ferrite core. The voltage measuringapparatus 114 is connected across the burden resistor 1204. The signalprocessing circuitry 116 is operative to receive an output from thevoltage measuring apparatus 114. The voltage measuring apparatus 114 isoperative to acquire the voltage developed across the burden resistor1204 and the signal processing circuitry 116 is operative to determinethe current signal, e.g. in respect of its peak and/or RMS value, independence on the acquired voltage and having regards to knowncharacteristics of the current transformer, e.g. the turns ratio of thecurrent transformer 1202 and the resistance of the burden resistor 1204.

The current measurement apparatus 1200 of FIG. 13 further comprises asignal source 112 of a form described above which is operative to pass areference input signal through a signal source conductor 1206. Thesignal source conductor 1206 passes through the coil 1202 of the currenttransformer. The reference input signal therefore induces acorresponding induced reference signal in the coil 1202. The signalsource 112 of FIG. 13 is therefore operative to modulate the currentsignal present in the coil 1202. The spatial separation amongst theelectrical circuit, the live conductor and the signal source conductoris maintained by their being bonded or held in the apparatus.Maintaining spatial separation is important for reducing misalignment ofmagnetic fields which may give rise to measurement inaccuracy. Thevoltage measuring apparatus 114 and the signal processing circuitry 116are operative to acquire the voltage signal developed across the burdenresistor by the load drawn current and reference input signals and toprocess the acquired voltage signal. More specifically the signalprocessing circuitry 116 is operative to extract a reference outputsignal corresponding to the reference input signal from the acquiredvoltage signal. The means of extraction is as described above.Characteristics of the current transformer, i.e. characteristics of thecoil 1202 and burden resistor 1204, and the processing chain aredetermined by comparison of the reference input signal applied by thesignal source 112 with the reference output signal. The determinedcharacteristics are stored. More specifically and with reference to thecoil the signal processing circuitry is operative to determine a phasedifference between the reference output signal and the reference inputsignal. The phase difference is used to compensate at least in part forphase error caused by the coil during ordinary operation when measuringthe load drawn current. Thus the characteristics determined for thecurrent transformer including the phase error are used to modify theload drawn current signal as measured by the voltage measuring apparatus114. As described above the signal source 112 is operative at spacedapart times to apply the reference input signal to the coil whencalibration of the current transformer and its processing chain isrequired or deemed appropriate. As can be seen from FIG. 13 a shuntresistor 410 is operative to measure current in the neutral conductor412 and the current transformer 1202 is operative to measure current inthe live conductor 404. The current transformer provides inherently forgalvanic isolation. Therefore the signal sources 112, 418 andacquisition and processing circuitry for each of the shunt resistor andcurrent transformer are constituted in the same integrated circuit 1208and without there being any need to provide for galvanic isolationbetween the shunt resistor circuitry and current transformer circuitry416. For example the comparison circuit 502, which is operative tocompare the current signals in the live and neutral conductors, receivesan output from shunt resistor signal processing circuitry 422 andcurrent transformer signal processing circuitry 116 without there beingany need, as per the circuit of FIG. 3B for example, to provide forisolation between the two outputs. In other embodiments of the inventionthe current measurement apparatus 1200 of FIG. 13 lacks the shuntresistor 410 and its processing chain such that the current measurementapparatus is operative to measure current in the live conductor 404 onlyby way of the current transformer 1202. In yet other embodiments thecurrent measurement apparatus 1200 is similarly configured and such thatthe current transformer is replaced with another form of current sensor,such as a Hall probe or a Rogowski coil, whereby the measurementapparatus is operative to measure current in the live conductor 404only.

Current measurement apparatus 1300 comprising a current transformer isshown in FIG. 14A. The current measurement apparatus 1300 is shownduring a tampering attempt involving application of an external magneticfield 1302. The current measurement apparatus 1300 comprises componentsin common with FIGS. 1A and 13. The reader's attention is thereforedirected to the description provided above with reference to FIGS. 1Aand 13 for a description of such common components. The signal source112 forms an integral part of the current measurement apparatus 1300,e.g. the signal source 112 is constituted with the rest of processingcircuitry of the current measurement apparatus 1300 as an integratedcircuit. In addition the current measurement apparatus 1300 comprises acentral processing unit (CPU) 1306 and a data store 1304, which areoperative to store characteristics determined for the currenttransformer over time. Such characteristics include transfer functioninformation for the current transformer and its processing chain.Application of the external magnetic field 1302 to the currentmeasurement apparatus 1300 interferes with proper operation of thecurrent transformer 1202. Such interference is reflected in an unduechange in the characteristics stored in the data store 1304 over time.The central processing unit 1306 is operative to monitor for such anundue change in characteristics and to provide an indication of an unduechange.

An alternative embodiment of current measurement apparatus 1320comprising a current transformer is shown in FIG. 14B. The currentmeasurement apparatus 1320 comprises components in common with FIG. 14A.The reader's attention is therefore directed to the description providedabove with reference to FIG. 14A for a description of such commoncomponents. In contrast with the circuit of FIG. 14A the signal source1324 and the signal source conductor 1326 are comprised in a separateunit to the rest of the processing circuitry of the current measurementapparatus. More specifically the signal source 1324 and the signalsource conductor 1326 form part of portable apparatus 1322 which isbrought into use, e.g. by maintenance or calibration personnel, whencalibration of the current transformer 1202 is required or when theoperation of the current measurement apparatus is being tested. When thecurrent measurement apparatus is operating normally the portableapparatus 1322 is removed.

Current measurement apparatus 1400 comprising a Rogowski coil is shownin FIG. 15. The current measurement apparatus 1400 comprises componentsin common with FIG. 14A. The reader's attention is therefore directed tothe description provided above with reference to FIG. 14A for adescription of such common components. In contrast with the circuit ofFIG. 14A the coil 1202 of the current transformer is replaced by aRogowski coil 1402, which is disposed around both live and neutralconductors 104, 106, and a burden resistor 1404 connected across theRogowski coil 1402. The Rogowski coil 1402 is operative to sense changeof current in the conductors with time. Accordingly the circuit 1400 ofFIG. 15 further comprises an integrator 1404 which is operative tointegrate the output from the voltage measuring apparatus 114 and applythe integrated signal to the signal processing circuitry 116. The signalsource conductor 1206, which carries the reference input signalgenerated by the signal source 112, passes through the Rogowski coil1402. Therefore the load current induced signal in the Rogowski coil1402 is modulated by the reference input signal in the same fashion asfor the current measurement apparatus comprising the current transformeras described above with reference to FIG. 14A. Otherwise the circuit ofFIG. 15 is operative as described above with reference to FIG. 14A.

Current measurement apparatus 1420 comprising a Hall effect probe 1422is shown in FIG. 16. The current measurement apparatus 1420 of FIG. 16comprises a Hall effect probe 1422 instead of the Rogowski coil of FIG.15. The Hall effect probe is disposed proximate the live wire 104 andthe signal source conductor 1206 whereby signals present in theseconductors are sensed by the Hall effect probe 1422. The currentmeasurement apparatus 1420 further comprises a Hall probe conditioningand measurement circuit 1424 which is operative to apply a bias currentto the Hall effect probe 1422 and to sample and acquire signals from theHall effect probe 1422 with the acquired signals being provided to asignal processing circuit 116. Otherwise the form and function of thecurrent measurement apparatus 1420 of FIG. 16 is as described elsewhereherein.

A first embodiment of current measurement apparatus 2000 which addressesthe challenge of extracting the reference output signal from the loadoutput signal is shown in FIG. 17A. Features in common with the currentmeasurement apparatus of FIG. 3A are indicated by the same referencenumerals and the reader's attention is directed to the descriptionprovided above with reference to FIG. 3A for a description of the formand function of such common features. Features particular to theembodiment of FIG. 17A will now be identified and their operationdescribed. The current measurement apparatus 2000 further comprises: asecond shunt resistor 2002 in series with the first shunt resistor 402in the live conductor 404; and a second voltage measuring apparatus2004, which is operative to acquire a voltage signal developed acrossthe second shunt resistor 2002. The output 2006 from the first voltagemeasuring apparatus 420 and the output 2008 from the second voltagemeasuring apparatus 2004 are both received by the signal processingcircuitry 422. The operation of the signal processing circuitry 422 isdescribed below. Outputs from the signal processing circuitry 422 arereceived by a compensation circuit 2010, which is operative as describedelsewhere herein to compensate for the initial value and drift in thetransfer function of the first shunt resistor 402 and its processingchain by determining a factor corresponding to a change in the transferfunction of the first shunt resistor 402 and its processing chain. Thesignal processing circuitry 422 comprises first 2012 and second 2014pre-processing circuitry which receive a respective one of the outputs2006, 2008 from the first and second voltage measuring apparatus 420,2004. The signal processing circuitry 422 further comprises astatistical operation block 2016, which receives inputs from the first2012 and second 2014 pre-processing circuitry, and an equaliser 2018,which receives inputs from the statistical operation block 2016 and thesecond pre-processing circuitry 2014. In addition the signal processingcircuitry 422 comprises a signal removal block 2020 which receivesinputs from the first pre-processing circuitry 2012 and the equaliser2018, an extraction block 2022 which receives an input from the signalremoval block 2020 and a combination block 2024 which receives inputsfrom the equaliser 2018 and direct from the first voltage measuringapparatus 420. The first and second shunt resistors 402, 2002 areconstituted as discrete components. Alternatively a single resistiveelement, such as a conductor in an integrated circuit or a wire in alead frame, is configured by way of a centre tap to provide for thefirst and second shunt resistors 402, 2002.

The operation of the current measurement apparatus 2000 of FIG. 17A willnow be described. The components of the present embodiment in commonwith the embodiment of FIG. 3A operate in the same fashion as previouslydescribed. Otherwise the second voltage measuring apparatus 2004 isoperative to acquire a voltage signal developed across the second shuntresistor 2002. The voltage signal developed across the second shuntresistor 2002 corresponds substantially to the current signal drawn bythe load 408, i.e. the voltage signal is the load output signal. Itshould be noted that no reference input signal is applied to the secondshunt resistor 2002. The first 2012 and second 2014 pre-processingcircuitry, the statistical operation block 2016 and the equaliser 2018are operative to receive the outputs 2006, 2008 from the first andsecond voltage measuring apparatus 420, 2004 and to equalise the outputswith respect to their amplitudes and phase. More specifically the first2012 and second 2014 pre-processing circuitry are operative to perform aFFT on the two outputs and the statistical operation block 2016 is thenoperative to analyse the resulting transformations to identify the mostsignificant frequency component which corresponds to the load drawncurrent signal and to analyse frequency components adjacent to thereference frequency component. Such analyses provide a basis fordetermining whether or not equalisation is required in respect of atleast one of phase and amplitude and, if so, the extent of theequalisation required. The equaliser 2018 is then operative to effectequalisation of the two outputs in dependence on the analyses by thestatistical operation block 2016 by operation on the output from thesecond pre-processing circuitry 2014. Thereafter the signal removalblock 2020 is operative to subtract the now equalised output from thesecond pre-processing circuitry 2014 (i.e. the equalised load outputsignal) from the output from the first pre-processing circuitry 2012 tothereby remove the load output signal and leave the reference outputsignal. The extraction block 2022 is then operative, as describedelsewhere herein, on the output from the signal removal block 2020 toextract the reference output signal from whatever noise there may be.The combination block 2024 is operative to combine the direct outputfrom the first voltage measuring apparatus 420 with the output from theequaliser 2018 to thereby combine signals acquired from the first andsecond shunt resistors. The signals are combined in respect of signalenergy outside the reference signal. Combining the signals in thisfashion improves the signal to noise ratio for frequencies other thanfrequencies of the reference input signal and thereby provides forimproved load drawn current measurement. The compensation circuit 2010is operative as described elsewhere to compensate for the transferfunction of the shunt resistor and its processing chain. The transferfunction includes a major component representative of the initial valueand drift in the impedance of the first shunt resistor 402. The signalprocessing circuitry 422 is further operative to determine whether ornot the load output signal is more than a predetermined amount, such as3 dB, above the noise floor and to selectively subtract the load outputsignal in dependence on the determination.

A second embodiment of current measurement apparatus 2100 which involvessubtraction of the load output signal is shown in FIG. 17B. Features incommon with the current measurement apparatus 2000 of FIG. 17A areindicated by the same reference numerals and the reader's attention isdirected to the description provided above with reference to FIG. 17Afor a description of the form and function of such common features.Features particular to the embodiment of FIG. 17B will now be identifiedand their operation described. The current measurement apparatus 2100 ofFIG. 17B comprises a current transformer 2102 in the neutral conductor412 instead of the second shunt resistor 2002 of FIG. 17A, which ispresent in the live conductor. A load drawn current present in the liveconductor normally returns by way of the neutral conductor. The currenttransformer 2102 is therefore operative to measure the returning loaddrawn current signal. The second voltage measuring apparatus 2004receives an input from the current transformer to thereby provide a loadoutput signal which is subtracted from a signal acquired by way of theshunt resistor 402. Otherwise the current measurement apparatus 2100 ofFIG. 17B is operative in the same fashion as the current measurementapparatus 2000 of FIG. 17A. It is however noted that the differentnatures of the first shunt resistor 402 and the current transformer 2102are liable to require a more complex equalisation process than isrequired where two components, such as two shunt resistors, which relyon the same current sensing principle are used. Where a shunt resistoris used in each of the live and neutral conductors instead of a shuntresistor and a current transformer the current measurement apparatus2100 further comprises an isolator as described elsewhere herein tomaintain isolation between the live and neutral conductors.

A third embodiment of current measurement apparatus 2200 which involvessubtraction of the load output signal is shown in FIG. 17C. Features incommon with the current measurement apparatus 2000 of FIG. 17A areindicated by the same reference numerals and the reader's attention isdirected to the description provided above with reference to FIG. 17Afor a description of the form and function of such common features.Features particular to the embodiment of FIG. 17C will now be identifiedand their operation described. The current measurement apparatus 2200comprises a modulated first signal source 2202, which is electricallycoupled across the shunt resistor 402, and a demodulator 2204, whichreceives an input from the voltage measuring apparatus 420 with thesignal processing circuitry 422 being operative to receive an input fromthe demodulator 2204. The modulated first signal source 2202 modulates areference input signal before application of the modulated referenceinput signal to the shunt resistor 402. Modulation of the referenceinput signal is by way of a form of Return To Zero (RTZ) approach or thelike. The demodulator 2204 is operative on the voltage signal 2206acquired by the voltage measuring apparatus 420 to separate the signal2206 output by the voltage measuring apparatus 420 into a first signal2208 comprising the reference output signal and the load output signaland a second signal 2210 comprising the load output signal andsubstantially lacking the reference output signal. After equalisation ofthe first and second signals as described above with reference to FIG.17A, the second signal is subtracted from the first signal to therebyobtain the reference output signal. In another form the modulation is ata sub-harmonic of the fundamental frequency of the load drawn signal andthe signal processing circuitry 422 is operative to extract informationfrom plural cycles of the load drawn signal. The current measurementapparatus 2200 of FIG. 17C is otherwise and thereafter operative asdescribed above with reference to FIG. 17A.

A fourth embodiment of current measurement apparatus 2300 which involvessubtraction of the load output signal is shown in FIG. 17D. Features incommon with the current measurement apparatus 2000 of FIG. 17A areindicated by the same reference numerals and the reader's attention isdirected to the description provided above with reference to FIG. 17Afor a description of the form and function of such common features.Features particular to the embodiment of FIG. 17D will now be identifiedand their operation described. The current measurement apparatus 2300 ofFIG. 17D comprises a second signal source 2302 which is operative toapply a second reference input signal to the second shunt resistor 2002.The second voltage measuring apparatus 2004 is operative to acquire avoltage signal developed across the second shunt resistor 2002. Thevoltage signal developed across the second shunt resistor 2002corresponds to the load drawn current signal and the second referenceinput signal. The voltage signal developed across the first shuntresistor 402, which is acquired by the first voltage measuring apparatus420 corresponds to the load drawn current signal and the first referenceinput signal. The output from each of the first and second voltagemeasuring apparatus is received by the signal processing circuitry 422.The first and second signal sources 418, 2302 are operative to apply thefirst and second reference signals at substantially the same time. Thefirst and second reference signals differ from each other with respectto at least one of frequency and phase. The signal processing circuitry422 is operative to equalise the voltage signals acquired by the firstand second voltage measuring circuits as described above with referenceto FIG. 17A. The signals acquired by the first and second voltagemeasuring circuits are subtracted from each other to thereby provide apair of output signals which substantially lack the load output signaland comprise the first and second reference output signals respectively.Otherwise the operation of the present embodiment is as described abovewith reference to FIG. 17A.

The invention claimed is:
 1. A current measurement apparatus comprising:a measurement arrangement configured to be disposed in relation to aload which draws a mains current signal, the measurement arrangementbeing operative when so disposed to measure the load drawn mains currentsignal; a signal source operative to apply a reference input signal tothe measurement arrangement whereby an output signal from themeasurement arrangement comprises a load output signal corresponding tothe load drawn mains current signal and a reference output signalcorresponding to the reference input signal; and a processing apparatuswhich is operative to receive the output signal and to make adetermination in dependence on the reference output signal and the loadoutput signal, the determination being in respect of at least one of theload drawn mains current signal and mains electrical power consumed bythe load.
 2. The current measurement apparatus according to claim 1 inwhich the processing apparatus is operative to determine at least oneof: a relationship between the reference input signal and the referenceoutput signal; and a change in a relationship between the referenceinput signal and the reference output signal.
 3. The current measurementapparatus according to claim 2 in which the processing apparatuscomprises data storage, the processing apparatus being operative: todetermine the relationship between the reference input signal and thereference output signal; and to store the determined relationship in thedata storage.
 4. The current measurement apparatus according to claim 3in which the relationship between the reference input signal and thereference output signal is determined upon manufacture of the currentmeasurement apparatus.
 5. The current measurement apparatus according toclaim 3 in which the determination made in dependence on the referenceoutput signal and the load output signal is based at least in part onthe relationship stored in the data storage.
 6. The current measurementapparatus according to claim 1 in which the processing apparatus isoperative: to determine a relationship between the reference inputsignal and the reference output signal; to determine a change betweenthe determined relationship and a relationship stored in data storagecomprised in the processing apparatus; and to compare the change with athreshold value.
 7. The current measurement apparatus according to claim6 in which the processing apparatus is operative to adjust determinationof at least one of the load drawn mains current signal and the mainselectrical power consumed by the load when the change between the storedrelationship and the determined relationship is greater than thethreshold value.
 8. The current measurement apparatus according to claim1 in which the processing apparatus is operative to adjust determinationof at least one of the load drawn mains current signal and the mainselectrical power consumed by the load in dependence on a relationshipbetween the reference input signal and the reference output signal andat least one of: temperature measurement; load drawn mains currentsignal measurement; and line voltage measurement.
 9. The currentmeasurement apparatus according to claim 1 in which the processingapparatus is operative to provide diagnostic data which at least one of:comprises a relationship between the reference input signal and thereference output signal; and is in dependence on a change between arelationship stored in data storage comprised in the processingapparatus and a relationship determined between the reference inputsignal and the reference output signal, the current measurementapparatus being configured to at least one of: output the diagnosticdata at the current measurement apparatus; and convey the diagnosticdata from the current measurement apparatus.
 10. The current measurementapparatus according to claim 9 in which the current measurementapparatus is configured to at least one of: display the diagnostic databy way of display apparatus comprised in current measurement apparatus;and convey the diagnostic data from the current measurement apparatus byway of network communication apparatus.
 11. The current measurementapparatus according to claim 1 in which the processing apparatus isoperative to determine a tampering event in dependence on a changebetween: a relationship determined between the reference input signaland the reference output signal; and a stored, previously determinedrelationship between the reference input signal and the reference outputsignal.
 12. The current measurement apparatus according to claim 1 inwhich the processing apparatus is operative to compare a relationshipbetween the reference input signal and the reference output signal witha threshold value and to determine a tampering event in dependence onthe comparison.
 13. The current measurement apparatus according to claim11 in which the tamper event is determined in dependence on at least oneof: temperature measurement; load drawn mains current signalmeasurement; and line voltage measurement.
 14. The current measurementapparatus according to claim 1 in which the measurement arrangementcomprises a current sensor, the current sensor comprising one of a shuntresistor, a current transformer, a Rogowski coil and a Hall Effectsensor.
 15. The current measurement apparatus according to claim 14 inwhich the current sensor is operative to sense current in dependence onelectromagnetic induction, the signal source comprises a signal sourceconductor which is operative to carry the reference input signal, andthe signal source conductor is disposed proximate the current sensorwhereby the reference input signal in the signal source conductorinduces an induced reference signal in the current sensor.
 16. Thecurrent measurement apparatus according to claim 1 in which the currentmeasurement apparatus is operative to determine a frequency dependenttransfer function of the measurement arrangement over a range offrequencies in dependence on the reference input signal comprising atleast one of: a frequency that changes over time; and differentfrequencies at any one time.
 17. The current measurement apparatusaccording to claim 1 in which the signal source is operative to apply tothe measurement arrangement a reference input signal having a knowncharacteristic which is substantially absent from the load drawn currentsignal and the processing apparatus is operative to learn which knowncharacteristic in the reference input signal is substantially absentfrom the load drawn current signal.
 18. The current measurementapparatus according to claim 1 comprising plural current sensors, thecurrent measurement apparatus being configured to provide galvanicisolation in a circuit path between the current sensors.
 19. The currentmeasurement apparatus according to claim 1 comprising first and secondsignal source reference circuits and the current measurement apparatusis configured to switch between the first and second signal sourcereference circuits such that the reference input signal is set by eachof the first and second signal source reference circuits.
 20. Thecurrent measurement apparatus according to claim 1 further comprising anextraction circuit and a subtraction circuit, the extraction circuitbeing operative to receive an output signal from the measurementarrangement and to extract the reference output signal from the receivedoutput signal, and the subtraction circuit being operative to subtractthe extracted reference output signal from an output from the processingapparatus to thereby leave the load output signal.
 21. The currentmeasurement apparatus according to claim 1 in which the measurementarrangement comprises a shunt resistor, the shunt resistor beingconstituted by a conductor forming part of or providing electricalconnectivity in or to an electrical component.
 22. The currentmeasurement apparatus according to claim 1 in which the reference inputsignal is at least one of: predetermined; and ascertained by measurementof the reference input signal.
 23. The current measurement apparatusaccording to claim 1 comprising: plural measurement arrangementsconfigured to be disposed in relation to a respective one of a neutralwire and at least one live wire of an electrical supply; at least onesignal source operative to apply a reference input signal to each of theplural measurement arrangements; and at least one processing apparatusoperative to determine the load drawn current signal passing througheach of the plural measurement arrangements or a transfer function inrespect of the each measurement arrangement.
 24. An electricityconsumption meter comprising the current measurement apparatus accordingto claim
 1. 25. A current measurement method comprising: applying areference input signal by way of a signal source to a measurementarrangement disposed in relation to a load which draws a mains currentsignal, the measurement arrangement being operative when so disposed tomeasure the load drawn mains current signal; receiving an output signalfrom the measurement arrangement in a processing apparatus, the outputsignal comprising a load output signal corresponding to the load drawnmains current signal and a reference output signal corresponding to thereference input signal; and making a determination in the processingapparatus in dependence on the reference output signal and the loadoutput signal, the determination being in respect of at least one of theload drawn mains current signal and mains electrical power consumed bythe load.